Thanks for the report. Working on this :)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8906ce8b964bf3431b891bc9eaaca54ad8981f7d

commit 8906ce8b964bf3431b891bc9eaaca54ad8981f7d
Author: zqzhang <zqzhang@chromium.org>
Date: Mon Dec 12 22:44:35 2016

Fix gmock leak issue in AutoplayUmaHelperTest

AutoplayUmaHelperTest currently uses GMock for mock
AutoplayUmaHelper in order to check if events are correctly
received. However this will make the test flaky since
MockAutoplyUmaHelper is garbage-collected but GMock expect it to
be destroyed when the test finishes.

This CL marks MockAutoplayUmaHelper as AllowLeak which fixes the
issue.

BUG= 672831 

Review-Url: https://codereview.chromium.org/2562683004
Cr-Commit-Position: refs/heads/master@{#437939}

[modify] https://crrev.com/8906ce8b964bf3431b891bc9eaaca54ad8981f7d/third_party/WebKit/Source/core/html/AutoplayUmaHelperTest.cpp

I see the flakiness is gone (even before I landed the fix...)
Let's keep it open for a couple of days and check again.

Flakiness dashboard:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_unit_tests&showAllRuns=true&tests=autoplay

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/99ac6d968116db3f8257044d3215fd222f9d7b81

commit 99ac6d968116db3f8257044d3215fd222f9d7b81
Author: zqzhang <zqzhang@chromium.org>
Date: Fri Dec 16 21:58:06 2016

Follow-up fix for leaking mock object in AutoplayUmaHelperTest

A previous attempt for fixing AutoplayUmaHelperTest uses
::testing::Mock::VerifyAndClear to clean up the expectations on the
mock. However it does not ignore the mock object to be leaked. This CL
solves the issue by calling ::testing::Mock::AllowLeak in SetUp().

BUG= 672831 

Review-Url: https://codereview.chromium.org/2580003002
Cr-Commit-Position: refs/heads/master@{#439205}

[modify] https://crrev.com/99ac6d968116db3f8257044d3215fd222f9d7b81/third_party/WebKit/Source/core/html/AutoplayUmaHelperTest.cpp