Actually we do have enough layout tests under: LayoutTests/animations/, we have tests for transform & opacity in there. 

vollick@, ajuma@: is my understanding correct?

Perhaps you could add a unit test that when the feature is enabled, animations don't get composited, and when it's disabled they do?

ajuma@: yes, that makes sense, will do. Thank you.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a

commit d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a
Author: Xida Chen <xidachen@chromium.org>
Date: Thu Oct 26 17:14:52 2017

Implement animation targeted UMA

This CL adds 3 animation specific UMAs. We first check the following 3
conditions:
1. Whether there is at least one composited animation.
2. Whether there is at least one main-thread animation.
3. Whether there is at least one main-thread animation, that it could
   be composited, but not due to running experiment(s).

For each of the above conditions, record the draw time.

The UMAs are designed for this experiment: crbug.com/754471, which is
to turn off compositing 2D transform & opacity animations, and we'd like
to have meaningful UMAs to measure the impact of the experiment.

The CL also added unit tests to make sure that:
1. We are getting the correct count for each of the above three
   conditions.
2. We are getting the correct UMA reporting.
3. In CompositorAnimationsTest.cpp, we added test case to make sure that
   the transform animation is composited with the
   "TurnOff2DAndOpacityCompositorAnimations" disabled, and not
   composited when the flag is enabled.

TBR=haraken@chromium.org

Bug:  758439 ,  761968 
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ie6eb207df86cb47b3670472d256f7a63bbe39914
Reviewed-on: https://chromium-review.googlesource.com/636305
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511859}
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/cc/animation/animation_host.cc
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/cc/animation/animation_host.h
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/cc/animation/animation_player.cc
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/cc/animation/animation_player.h
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/cc/animation/animation_ticker.cc
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/cc/animation/animation_ticker.h
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/cc/animation/element_animations_unittest.cc
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/cc/scheduler/compositor_timing_history.cc
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[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/core/animation/Animation.h
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/core/animation/CompositorAnimations.cpp
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/core/animation/CompositorAnimations.h
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/core/animation/CompositorAnimationsTest.cpp
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/core/animation/DocumentAnimations.cpp
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[add] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/core/animation/test_data/transform-animation.html
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/platform/animation/CompositorAnimationHost.cpp
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/platform/animation/CompositorAnimationHost.h
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/platform/testing/RuntimeEnabledFeaturesTestHelpers.h
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/platform/testing/URLTestHelpers.cpp
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/platform/testing/URLTestHelpers.h
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/tools/metrics/histograms/histograms.xml