The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4e2700ead18fc989d70978adfacef881b44e8675

commit 4e2700ead18fc989d70978adfacef881b44e8675
Author: Xida Chen <xidachen@chromium.org>
Date: Fri Mar 23 23:08:21 2018

Fix a mistake in computing main thread compositable animations

At this moment, this is how we check whether an animation is main thread
compositable or not:
1. The runtime feature TurnOff2DAndOpacityCompositorAnimationsEnabled is on
2. There is either an opacity or 2d transform animation

There is a problem with this logic. When there is an opacity animation
on an element and the element has "will-change: transform", we will put
this animation as main thread compositable. This is not true, we should
categorize it as composited animation because "will-change: transform"
will create a layer anyways.

This CL fixes this problem. We care about the most common cases:
"will-change: transform" and 3D transform. So step 2 in the above
becomes: when there is no "will-change: transform" and there is no 3D
transform, and there is either opacity or 2D transform, then this is a
main thread compositable animation.

This CL also added a test to verify that an element with
"will-change: transform" + opacity animation will end up having a
composited animation even when the runtime feature is on.

Bug:  818809 
Change-Id: Ifcd027223df96c17447955eb52568d74bb5505ff
Reviewed-on: https://chromium-review.googlesource.com/951719
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545612}
[modify] https://crrev.com/4e2700ead18fc989d70978adfacef881b44e8675/third_party/WebKit/Source/core/animation/CompositorAnimations.cpp
[modify] https://crrev.com/4e2700ead18fc989d70978adfacef881b44e8675/third_party/WebKit/Source/core/animation/CompositorAnimations.h
[modify] https://crrev.com/4e2700ead18fc989d70978adfacef881b44e8675/third_party/WebKit/Source/core/animation/CompositorAnimationsTest.cpp
[add] https://crrev.com/4e2700ead18fc989d70978adfacef881b44e8675/third_party/WebKit/Source/core/animation/test_data/opacity-with-3d-transform.html
[add] https://crrev.com/4e2700ead18fc989d70978adfacef881b44e8675/third_party/WebKit/Source/core/animation/test_data/opacity-with-will-change-transform.html