This also generates a lot of noise in browser_tests, so it's not just AppShell:

$ ./out_linux/rel/browser_tests --gtest_filter=TaskManagerBrowserTest.ReloadExtension
IMPORTANT DEBUGGING NOTE: each test is run inside its own process.
For debugging a test inside a debugger, use the
--gtest_filter=<your_test_name> flag along with either
--single_process (to run the test in one launcher/browser process) or
--single-process (to do the above, and also run Chrome in single-process mode).
Using sharding settings from environment. This is shard 0/1
Using 1 parallel jobs.
Note: Google Test filter = TaskManagerBrowserTest.ReloadExtension
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from TaskManagerBrowserTest, where TypeParam =
[ RUN      ] TaskManagerBrowserTest.ReloadExtension
[91858:91858:0822/175042.806316:WARNING:password_store_factory.cc(241)] Using basic (unencrypted) store for password storage. See https://chromium.googlesource.com/chromium/src/+/master/docs/linux_password_storage.md for more information about password storage options.
[91858:91899:0822/175042.871214:WARNING:child_process_launcher_helper_posix.cc(118)] Ignoring invalid file v8_context_snapshot.bin

(browser_tests:91858): LIBDBUSMENU-GLIB-WARNING **: Unable to get session bus: Unknown or unsupported transport 'disabled' for address 'disabled:'
[91858:91899:0822/175043.962354:WARNING:child_process_launcher_helper_posix.cc(118)] Ignoring invalid file v8_context_snapshot.bin
[91858:91899:0822/175044.176433:WARNING:child_process_launcher_helper_posix.cc(118)] Ignoring invalid file v8_context_snapshot.bin
[91858:91899:0822/175044.333468:WARNING:child_process_launcher_helper_posix.cc(118)] Ignoring invalid file v8_context_snapshot.bin
[91858:91899:0822/175044.791660:WARNING:child_process_launcher_helper_posix.cc(118)] Ignoring invalid file v8_context_snapshot.bin
[91858:91899:0822/175044.958828:WARNING:child_process_launcher_helper_posix.cc(118)] Ignoring invalid file v8_context_snapshot.bin
[91858:91899:0822/175045.119331:WARNING:child_process_launcher_helper_posix.cc(118)] Ignoring invalid file v8_context_snapshot.bin
[91858:91858:0822/175045.308032:INFO:chrome_cryptauth_service.cc(222)] Profile is not authenticated yet; waiting before starting CryptAuth managers.
[91858:91858:0822/175045.309921:ERROR:event_page_request_manager.cc(153)] An error encountered while waking the event page.
[91858:91858:0822/175045.309949:ERROR:event_page_request_manager.cc(95)] Draining request queue. (queue-length=2)
[       OK ] TaskManagerBrowserTest.ReloadExtension (3213 ms)
[----------] 1 test from TaskManagerBrowserTest (3213 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (3214 ms total)
[  PASSED  ] 1 test.
[1/1] TaskManagerBrowserTest.ReloadExtension (3931 ms)
SUCCESS: all tests passed.
(

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5338611c0a7ae76c19a4800f1d45aec943dbed2f

commit 5338611c0a7ae76c19a4800f1d45aec943dbed2f
Author: Hitoshi Yoshida <peria@chromium.org>
Date: Wed Aug 23 08:29:03 2017

bindings: Bind v8_context_snapshot.bin to browser_tests

Bug:  757956 
Change-Id: I648e9d706bc9aa6582447c4b319e804f155a948e
Reviewed-on: https://chromium-review.googlesource.com/627531
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496625}
[modify] https://crrev.com/5338611c0a7ae76c19a4800f1d45aec943dbed2f/content/test/BUILD.gn
[modify] https://crrev.com/5338611c0a7ae76c19a4800f1d45aec943dbed2f/tools/v8_context_snapshot/BUILD.gn

Michael,
Thank you for reporting this.  I think #3 works for browser_tests.
But I can't reproduce the message in app_shell on MacOSX.
Could you tell me which you think strange in .gn files?
I think app_shell depends on app_shell_framework_bundle_data, and it has the bin file in source.

app_shell on macOS specifically lists v8_context_snapshot.bin in its deps, but app_shell on Linux/Windows/CrOS do not. So the app_shell warning reproduces on Linux and, I assume, Windows and CrOS too.

Shouldn't it be a dependency for every platform, not just Mac?

Yes, app_shell on other platforms also need to depend on it. Lacking the deps parameter is my mistake.

I'm not sure which CL actually works, but for now, the warning does not show up.
So let me close this. If you think it is still going, please let me know.