According to https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=AnimationEffect-getComputedTiming.tentative.html, the two AnimationEffect-getComputedTiming.tentative.html tests are failing quite consistently on most builders.

Disabling them with https://crrev.com/c/1181044.

Latest failure:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.13%20Tests%20%28dbg%29/4807

Example log:
02:38:37.377 25315 worker/0 external/wpt/css/css-animations/AnimationEffect-getComputedTiming.tentative.html failed:
02:38:37.377 25315 worker/0  asserts failed
02:38:37.377 25315 worker/0 killing secondary driver
02:38:37.378 25315 worker/0 exiting
02:38:37.378 25315 worker/0 cleaning up
02:38:37.378 25315 worker/0 killing primary driver
02:38:37.378 25315 killed pid 26731
02:38:37.379 25315 worker/0 killing secondary driver
02:38:37.380 25315 
02:38:37.380 25315 Retrying 1 unexpected failure, attempt 3 of 3...
02:38:37.380 25315 Sharding tests ...
02:38:37.380 25315 Starting 1 worker ...
02:38:37.381 25315 worker/0 starting
02:38:37.797 25315 "system_profiler SPDisplaysDataType" took 0.42s
02:38:40.209 25315 worker/0 external/wpt/css/css-animations/AnimationEffect-getComputedTiming.tentative.html output stderr lines:
02:38:40.210 25315   
02:38:40.210 25315   DevTools listening on ws://127.0.0.1:60379/devtools/browser/ec3ee695-ff54-48f4-a33b-6068095bd437
02:38:40.210 25315   objc[26737]: Class MockCrApp is implemented in both /b/s/w/ir/out/Debug/libtest_runner.dylib (0x1442be960) and /b/s/w/ir/out/Debug/Content Shell.app/Contents/Frameworks/Content Shell Framework.framework/Versions/C/Content Shell Framework (0x10968c088). One of the two will be used. Which one is undefined.
02:38:40.210 25315   objc[26738]: Class MockCrApp is implemented in both /b/s/w/ir/out/Debug/libtest_runner.dylib (0x13e94c960) and /b/s/w/ir/out/Debug/Content Shell.app/Contents/Frameworks/Content Shell Framework.framework/Versions/C/Content Shell Framework (0x106bcc088). One of the two will be used. Which one is undefined.
02:38:40.210 25315   [26737:775:0820/023839.443268:WARNING:vt_video_decode_accelerator_mac.cc(193)] Failed to create VTDecompressionSession: Error Domain=NSOSStatusErrorDomain Code=-8973 "codecOpenErr" (-8973)
02:38:40.210 25315   [26737:775:0820/023839.443947:WARNING:vt_video_decode_accelerator_mac.cc(215)] Hardware decoding with VideoToolbox is not supported
02:38:40.210 25315   [26737:775:0820/023839.477380:ERROR:vt_video_encode_accelerator_mac.cc(512)]  VTCompressionSessionCreate failed: -12908
02:38:40.212 25315 [1/1] external/wpt/css/css-animations/AnimationEffect-getComputedTiming.tentative.html failed unexpectedly (asserts failed)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/189cf3530ee8aa102e4d1703d084d71923f56520

commit 189cf3530ee8aa102e4d1703d084d71923f56520
Author: Friedrich Horschig <fhorschig@chromium.org>
Date: Mon Aug 20 13:15:07 2018

[Flaky] Mark newly imported WPT tests as flaky

As described in the linked bug, these tests should rather marked failing
than reverting the rollout, so doing this now.
They are failing _quite_ consistently but pass from time to time.

TBR=robertma@chromium.org

Bug: 875622
Change-Id: I34defdcd7ccda264b04acaf0ca90601668d35f80
Reviewed-on: https://chromium-review.googlesource.com/1181044
Reviewed-by: Friedrich Horschig <fhorschig@chromium.org>
Commit-Queue: Friedrich Horschig <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584420}
[modify] https://crrev.com/189cf3530ee8aa102e4d1703d084d71923f56520/third_party/WebKit/LayoutTests/TestExpectations

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/50d9f1fe959138caffd7d619d4a99d8aa029a55c

commit 50d9f1fe959138caffd7d619d4a99d8aa029a55c
Author: Philip Jägenstedt <foolip@chromium.org>
Date: Wed Aug 22 13:33:15 2018

Mark an incoming wpt test (CSSTransition-startTime.tentative.html) as flaky

https://chromium-review.googlesource.com/c/chromium/src/+/1184510 is
the most recent failure to import due to this test.

This test fails passes on try bots but fails in CQ for Linux and
Win. Include timeout in expectations too since that is the current
state.

TBR=robertma

Bug: 875622
Change-Id: I24662da5bf918941e59bd331a7871050c798d177
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1184904
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584998}
[modify] https://crrev.com/50d9f1fe959138caffd7d619d4a99d8aa029a55c/third_party/WebKit/LayoutTests/TestExpectations

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/62ecd8c326b32dcdd92c3f0c45b39b1d5e2a5abd

commit 62ecd8c326b32dcdd92c3f0c45b39b1d5e2a5abd
Author: Majid Valipour <majidvp@chromium.org>
Date: Thu Jan 17 19:13:17 2019

[transitions] Update CSSTransition-startTime.tentative test expectation

The CSSTransition-startTime.tentative.html test has been updates upstream
to fail consistently with new timeouts.

Since Blink does not implement 'transition{start,end}' events (http://crbug.com/439056)
the event tests are expected to fail. The expectation file reflects this
so the test should no longer timeout or be flaky.

Local test (20 runs) suggests it is consistent now. Updating expectation
file to reflect this.


R: smcgruer@chromium.org
Bug: 875622, 922970
Change-Id: Ica3999434e7126934b70dfc5bd755f8c45904d62
Reviewed-on: https://chromium-review.googlesource.com/c/1416879
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623785}
[modify] https://crrev.com/62ecd8c326b32dcdd92c3f0c45b39b1d5e2a5abd/third_party/blink/web_tests/TestExpectations