I was actually just about to file a nearly identical bug to this because I ran into exactly this problem in https://codereview.chromium.org/2066703004/. I added a FeatureList and some unit tests that use it, but on iOS only, I was getting failures in unrelated unit tests.

It turns out that, at least in component_unittests, this global state is not reset between test runs, and my feature was still turned on for later, unrelated test runs, which was causing the tests to fail. As a temporary measure, I'm going to have to explicitly clear out the instance's FeatureList before the test starts, but this was unexpected, to say the least.

Annoyingly, just calling ClearInstanceForTesting doesn't seem to be right either. That leaves the instance as NULL and so later tests will crash on FeatureList::IsEnabled. :-/

Agree that we should fix this. I can take this when I'm back from vacation - which will be in a couple of weeks, or happy to let someone else drive this. Should be pretty straight forward, I think.

Thanks, asvitkine@! I'll assign to you for triage purposes then.

davidben@: Indeed, I learned that the hard way as well. My solution in https://codereview.chromium.org/2066703004/ is to ClearInstanceForTesting() and then immediately allocate a new FeatureList and pass it to SetInstance().

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9499b8d28ace546ebe511b777fa3529421ecc256

commit 9499b8d28ace546ebe511b777fa3529421ecc256
Author: asvitkine <asvitkine@chromium.org>
Date: Tue Aug 09 05:37:07 2016

Add a ScopedFeatureList class for testing and start using it.

This class improves upon the current situation by ensuring a test
doesn't end up modifying the global feature list permanently.

Migrates a number of tests to new the scoped class.

BUG= 620435 
TBR=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2226063002
Cr-Commit-Position: refs/heads/master@{#410589}

[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/base/feature_list.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/base/feature_list.h
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/base/test/BUILD.gn
[add] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/base/test/scoped_feature_list.cc
[add] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/base/test/scoped_feature_list.h
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/chrome/browser/password_manager/chrome_password_manager_client_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/autofill/core/browser/autofill_assistant_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/autofill/core/browser/autofill_manager_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/autofill/core/browser/autofill_merge_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/autofill/core/browser/personal_data_manager_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/browser_sync/browser/profile_sync_service_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/flags_ui/BUILD.gn
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/flags_ui/flags_state_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/network_time/network_time_tracker_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/offline_pages/offline_page_model_impl_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/password_manager/core/browser/password_form_manager_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/password_manager/core/browser/password_manager_test_utils.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/password_manager/core/browser/password_manager_test_utils.h
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/password_manager/core/browser/password_manager_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/password_manager/sync/browser/sync_credentials_filter_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/ssl_config/BUILD.gn
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/ssl_config/ssl_config_service_manager_pref_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/subresource_filter/core/browser/BUILD.gn
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/subresource_filter/core/browser/subresource_filter_features_test_support.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/subresource_filter/core/browser/subresource_filter_features_test_support.h
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/translate/core/browser/translate_prefs_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/translate/core/browser/translate_ui_delegate_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/variations/service/variations_service_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/variations/variations_associated_data_unittest.cc
[modify] https://crrev.com/9499b8d28ace546ebe511b777fa3529421ecc256/components/variations/variations_seed_processor_unittest.cc

Keeping this bug open as I plan to do another pass and convert some more tests to use the new scoped class.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b1db826c74369f712111099190ae6bc6311ec9e7

commit b1db826c74369f712111099190ae6bc6311ec9e7
Author: asvitkine <asvitkine@chromium.org>
Date: Tue Nov 08 09:48:20 2016

Migrate more tests to ScopedFeatureList.

ScopedFeatureList ensures that the global FeatureList
instance is restored to the original state at the end
of the scope and results in less code.

BUG= 620435 
TBR=brettw@chromium.org

Review-Url: https://codereview.chromium.org/2484613002
Cr-Commit-Position: refs/heads/master@{#430560}

[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/base/feature_list.h
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/android/offline_pages/offline_page_utils_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/browser_about_handler_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/chromeos/hats/hats_finch_helper_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/extensions/extension_service_sync_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/metrics/antivirus_metrics_provider_win_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/password_manager/password_manager_browsertest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/safe_browsing/incident_reporting/incident_reporting_service_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/safe_browsing/permission_reporter_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/ssl/chrome_expect_ct_reporter_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/supervised_user/supervised_user_service_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/ui/startup/default_browser_infobar_delegate_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/ui/views/constrained_window_views_browsertest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/ui/views/passwords/manage_passwords_bubble_view_interactive_uitest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/ui/views/translate/translate_bubble_view_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/browser/ui/webui/uber/uber_ui_browsertest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/chrome/renderer/autofill/password_autofill_agent_browsertest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/components/sync/engine_impl/sync_manager_impl_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/components/variations/field_trial_config/BUILD.gn
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/components/variations/field_trial_config/field_trial_util_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/content/browser/loader/async_resource_handler_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/content/browser/loader/async_revalidation_manager_browsertest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/content/browser/loader/reload_cache_control_browsertest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/content/browser/loader/resource_dispatcher_host_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/content/browser/media/session/media_session_impl_visibility_browsertest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/content/browser/memory/memory_coordinator.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/content/browser/memory/memory_coordinator.h
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/content/browser/memory/memory_coordinator_browsertest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/content/browser/memory/memory_coordinator_impl_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/content/child/resource_dispatcher_unittest.cc
[modify] https://crrev.com/b1db826c74369f712111099190ae6bc6311ec9e7/media/blink/webmediaplayer_impl_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0e9bca93e104ad63ea99ce376c6923182cf0c803

commit 0e9bca93e104ad63ea99ce376c6923182cf0c803
Author: asvitkine <asvitkine@chromium.org>
Date: Mon Nov 21 20:38:38 2016

Migrate md history test code to ScopedFeatureList.

ScopedFeatureList ensures that the previous FeatureList
is restored at the end of the test, which the previous
code did not do.

BUG= 620435 

Review-Url: https://codereview.chromium.org/2511993002
Cr-Commit-Position: refs/heads/master@{#433633}

[modify] https://crrev.com/0e9bca93e104ad63ea99ce376c6923182cf0c803/chrome/browser/history/history_browsertest.cc
[modify] https://crrev.com/0e9bca93e104ad63ea99ce376c6923182cf0c803/chrome/browser/ui/find_bar/find_bar_host_browsertest.cc
[modify] https://crrev.com/0e9bca93e104ad63ea99ce376c6923182cf0c803/chrome/browser/ui/webui/browsing_history_handler_unittest.cc
[modify] https://crrev.com/0e9bca93e104ad63ea99ce376c6923182cf0c803/chrome/browser/ui/webui/log_web_ui_url_browsertest.cc
[modify] https://crrev.com/0e9bca93e104ad63ea99ce376c6923182cf0c803/chrome/browser/ui/webui/md_history_ui.cc
[modify] https://crrev.com/0e9bca93e104ad63ea99ce376c6923182cf0c803/chrome/browser/ui/webui/md_history_ui.h
[modify] https://crrev.com/0e9bca93e104ad63ea99ce376c6923182cf0c803/chrome/browser/ui/webui/uber/uber_ui_browsertest.cc