Findit identified the culprit r561518 with confidence 70.0% in the config "chromium.memory / Linux ChromiumOS MSan Tests"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVyiQILEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCLSAWNocm9taXVtLm1lbW9yeS9MaW51eCBDaHJvbWl1bU9TIE1TYW4gVGVzdHMvNzI4OS9pbnRlcmFjdGl2ZV91aV90ZXN0cy9RV3hzTDFCaGMzTjNiM0prVFdGdVlXZGxja0p5YjNkelpYSlVaWE4wVjJsMGFFTnZibVJwZEdsdmJtRnNVRzl3ZFhCV2FXVjNjeTVRY205dGNIUkdiM0pHWlhSamFGZHBkR2hPWlhkUVlYTnpkMjl5WkhOWGFYUm9iM1YwVDI1VGRXSnRhWFF2TVE9PQwLEhNNYXN0ZXJGbGFrZUFuYWx5c2lzGAEM


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r561518 is

Navigation: Recycling commit verification in tests

author	Arthur Hemery <ahemery@chromium.org>	Thu May 24 16:20:53 2018

Currently tests that check that a RFH tried to commit in unit tests used
MockRenderProcessHost::did_frame_commit_navigation(). This was set by
intercepting the interface commit using
TestRenderFrameHost::NavigationInterceptor. This is problematic because in the
future the RFH will not be the one holding the interface directly, making it
harder to use this mechanism. Instead, we just check that the RFH has a
navigation request, which is equivalent to it trying to commit.

We also remove TestRenderFrameHost::pending_commit() that was never used and
now redundant.



likely related is an error of 
All/PasswordManagerBrowserTestWithConditionalPopupViews.AutofillLoginSignupForm/1

that goes

../../chrome/browser/password_manager/password_manager_test_base.cc:640: Failure
Expected equality of these values:
  expected_value
    Which is: "user"
  return_value
    Which is: ""
element_id = username
Stack trace:
    #0 0x00000171d6b7 in StackTraceGetter::CurrentStackTrace(int, int) ./../../third_party/googletest/custom/gtest/internal/custom/stack_trace_getter.cc:22:27
    #1 0x0000017567c9 in testing::internal::UnitTestImpl::CurrentOsStackTraceExceptTop(int) ./../../third_party/googletest/src/googletest/src/gtest.cc:810:35
    #2 0x000001754922 in testing::internal::AssertHelper::operator=(testing::Message const&) const ./../../third_party/googletest/src/googletest/src/gtest.cc:386:25
    #3 0x0000235a3876 in PasswordManagerBrowserTestBase::CheckElementValue(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) ./../../chrome/browser/password_manager/password_manager_test_base.cc:640:3
    #4 0x0000235a2bc5 in PasswordManagerBrowserTestBase::CheckElementValue(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) ./../../chrome/browser/password_manager/password_manager_test_base.cc:620:3
    #5 0x0000008f00b1 in PasswordManagerInteractiveTestBase::VerifyPasswordIsSavedAndFilled(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) ./../../chrome/browser/password_manager/password_manager_interactive_test_base.cc:55:5
    #6 0x000000904f0f in password_manager::PasswordManagerBrowserTestWithConditionalPopupViews_AutofillLoginSignupForm_Test::RunTestOnMainThread() ./../../chrome/browser/password_manager/password_manager_interactive_uitest.cc:285:3
    #7 0x00000e4fe3b2 in content::BrowserTestBase::ProxyRunTestOnMainThreadLoop() ./../../content/public/test/browser_test_base.cc:385:5


https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.memory%2FLinux_ChromiumOS_MSan_Tests%2F7291%2F%2B%2Frecipes%2Fsteps%2Finteractive_ui_tests%2F0%2Flogs%2FAll__x2f_PasswordManagerBrowserTestWithConditionalPopupViews.AutofillLoginSignupForm__x2f_1%2F0


That's a tenuous link.. The tree is a horrible mess - expect these to be disabled so I can restore my sanity.

Hi, This is an assigned Sheriff-Chromium bug. You're getting this update because the sheriff didn't have time to investigate/poke this bug today. You can help by taking appropriate action. E.g.
 - fix \o/
 - disable test and remove Sheriff-Chromium label
 - revert a CL (and remove label)
 - re-triage/de-dupe, removing Sheriff-Chromium if you think it's no longer flaky.

Thanks!

I don't think this is caused by Arthur's CL, which modified content_unittests. I don't see how that would affect browser_tests.

It was me who introduced this line to get a better failure description
https://chromium-review.googlesource.com/c/chromium/src/+/1071668