I'm marking the test as flaky in https://crrev.com/c/951788. I suspect r541225 is the culprit, but I'm not confident enough to revert it. wangxianzhu@, can you please take a look and re-assign if this is not related to your change?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ab595088ec2dd55b76cae534c86af4769a687128

commit ab595088ec2dd55b76cae534c86af4769a687128
Author: jdoerrie <jdoerrie@chromium.org>
Date: Wed Mar 07 12:35:04 2018

[webkit] Mark composited-with-hit-testing.html as flaky

TBR=wangxianzhu

Bug:  819591 
Change-Id: I5580ac037f5d6634a5d76708a0ff8c7dbd73c53a
Reviewed-on: https://chromium-review.googlesource.com/951788
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541413}
[modify] https://crrev.com/ab595088ec2dd55b76cae534c86af4769a687128/third_party/WebKit/LayoutTests/TestExpectations

Locally bisected to https://chromium-review.googlesource.com/941963.

Looking now.

Well, certainly seems to be my change and I can't immediately answer why; reverting.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/71d23d568f857be5095346c47bfc8fd3aea3448e

commit 71d23d568f857be5095346c47bfc8fd3aea3448e
Author: Stephen McGruer <smcgruer@chromium.org>
Date: Wed Mar 07 20:54:53 2018

Revert "Web Animations: use WTF::Optional for Animation::start_time_"

This reverts commit c838bf406fc8f97bd8c26a713f80e7290c1f39d3.

Reason for revert: virtual/threaded/animations/hit-testing/composited-with-hit-testing.html fails in Debug, flaky in Release

Builders failed on: 
- WebKit Linux Trusty (dbg): 
 https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20Trusty%20%28dbg%29

- Win7 Tests (dbg)(1):   https://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%28dbg%29%281%29

Flakiness Dashboard:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=virtual%2Fthreaded%2Fanimations%2Fhit-testing%2Fcomposited-with-hit-testing.html

Bug:  819591 

Original change's description:
> Web Animations: use WTF::Optional for Animation::start_time_
> 
> Bug: 791086
> Change-Id: I916508fc5cf0c16543decd3f8197d034f2a4e6d8
> Reviewed-on: https://chromium-review.googlesource.com/941963
> Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
> Reviewed-by: Robert Flack <flackr@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#541223}

TBR=flackr@chromium.org,smcgruer@chromium.org

Change-Id: I2094317fbf67d28bb5c32124a7d3fa62c7323a3b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 791086
Reviewed-on: https://chromium-review.googlesource.com/953363
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541572}
[modify] https://crrev.com/71d23d568f857be5095346c47bfc8fd3aea3448e/third_party/WebKit/Source/core/animation/Animation.cpp
[modify] https://crrev.com/71d23d568f857be5095346c47bfc8fd3aea3448e/third_party/WebKit/Source/core/animation/Animation.h
[modify] https://crrev.com/71d23d568f857be5095346c47bfc8fd3aea3448e/third_party/WebKit/Source/core/animation/AnimationTest.cpp
[modify] https://crrev.com/71d23d568f857be5095346c47bfc8fd3aea3448e/third_party/WebKit/Source/core/animation/PendingAnimations.cpp
[modify] https://crrev.com/71d23d568f857be5095346c47bfc8fd3aea3448e/third_party/WebKit/Source/core/animation/css/CSSAnimations.cpp
[modify] https://crrev.com/71d23d568f857be5095346c47bfc8fd3aea3448e/third_party/WebKit/Source/core/inspector/InspectorAnimationAgent.cpp

Dropping to P3 now that merge is in. Will attempt to root-cause reason and re-land the CL sometime later this week.

Sheriffs - please keep an eye on the dashboard in-case the test continues to flake after the revert.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3d66e436c024f8797ecec026fe3d8028c097a4f2

commit 3d66e436c024f8797ecec026fe3d8028c097a4f2
Author: Stephen McGruer <smcgruer@chromium.org>
Date: Mon Mar 19 19:18:45 2018

Revert "[webkit] Mark composited-with-hit-testing.html as flaky"

This reverts commit ab595088ec2dd55b76cae534c86af4769a687128.

Reason for revert: With https://chromium.googlesource.com/chromium/src/+/c838bf406fc8f97bd8c26a713f80e7290c1f39d3 reverted, this is no longer broken or flaky.

Original change's description:
> [webkit] Mark composited-with-hit-testing.html as flaky
> 
> TBR=wangxianzhu
> 
> Bug:  819591 
> Change-Id: I5580ac037f5d6634a5d76708a0ff8c7dbd73c53a
> Reviewed-on: https://chromium-review.googlesource.com/951788
> Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
> Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#541413}

TBR=wangxianzhu@chromium.org,jdoerrie@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug:  819591 
Change-Id: I47f361c527f7e29e7c4b88ec2207704e026fcc8b
Reviewed-on: https://chromium-review.googlesource.com/962821
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544105}
[modify] https://crrev.com/3d66e436c024f8797ecec026fe3d8028c097a4f2/third_party/WebKit/LayoutTests/TestExpectations

The issue was root-caused and a reland CL is in review. I'm going to leave this bug open until it lands, though the actual test was already fixed by the revert (and re-enabled a week later when I remembered!).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/59f06466fb092c256db7fbe4c392079ddf13e96a

commit 59f06466fb092c256db7fbe4c392079ddf13e96a
Author: Stephen McGruer <smcgruer@chromium.org>
Date: Wed Mar 21 13:25:28 2018

Reland "Web Animations: use WTF::Optional for Animation::start_time_"

This is a reland of c838bf406fc8f97bd8c26a713f80e7290c1f39d3

The original CL contained two subtle bugs from a comparison of
compositor_state_->start_time to start_time_. Previously if both were
'null' (represented by NaN), the comparison would still say they were
inequal because NaN != NaN. The original CL 'fixed' those comparisons,
but it turns out the behavior was intentional. This version of the CL
restores the intentional behavior, adds a test for it, and documents it.

Bug: 791086,  819591 
Change-Id: I6587d028e66c6f6a9213b60f8d5f51b1dac3f899
Reviewed-on: https://chromium-review.googlesource.com/956280
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544679}
[modify] https://crrev.com/59f06466fb092c256db7fbe4c392079ddf13e96a/third_party/WebKit/Source/core/animation/Animation.cpp
[modify] https://crrev.com/59f06466fb092c256db7fbe4c392079ddf13e96a/third_party/WebKit/Source/core/animation/Animation.h
[modify] https://crrev.com/59f06466fb092c256db7fbe4c392079ddf13e96a/third_party/WebKit/Source/core/animation/AnimationTest.cpp
[modify] https://crrev.com/59f06466fb092c256db7fbe4c392079ddf13e96a/third_party/WebKit/Source/core/animation/PendingAnimations.cpp
[modify] https://crrev.com/59f06466fb092c256db7fbe4c392079ddf13e96a/third_party/WebKit/Source/core/animation/css/CSSAnimations.cpp
[modify] https://crrev.com/59f06466fb092c256db7fbe4c392079ddf13e96a/third_party/WebKit/Source/core/inspector/InspectorAnimationAgent.cpp