Note: discussed the above with flackr at length today: thanks for all the insights
about how to achieve this.

Another note on implementation: Blink can communicate the main-thread state about
running animations to cc::AnimationHost by way of a method on blink::CompositedAnimationHost, which is owned by the root LocalFrameView.

I think ideally we would have a count of the number of main thread and composited animations so that we might better gauge the impact of this change. I.e. there are some sites which currently have a main thread background color animation but smooth transform animations which may look worse when those transforms are moved to main as well. This is of course very hard to judge since any particular animation could be more visually jarring than another.

The activation timing should be recorded at draw time as well; in CompositorTimingHistory::DidDraw we know if we used a new active tree. This indicates that main thread animations would have updated and we'll be comparing the same times (when the compositor finished the frame) rather than comparing different events.

This related to (perhaps a more detailed subset) of tdresser's effort to define a high-level frame throughput metric -  issue 625700 .

Had a meeting with brianderson@ and vmiura@, it appears that there are some overlap between this and the work in GPU team that tries to measure frame throughput.

Currently I have a WIP CL for this bug, does it make sense to put the CL on hold until we have a final plan for how to measure the frame throughput?

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
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[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.cpp
[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
[modify] https://crrev.com/d8bb3b2e9c6fb3b28cd4be68b1c5fbece8d4900a/third_party/WebKit/Source/core/animation/DocumentTimeline.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