PowerMode: Track main thread animations separately

Adds a separate PowerModeVoter to cc::Scheduler to identify when the
main thread is producing frames.

This helps us better identify no-op animations that involve the main
thread (e.g. rAF), while still distinguishing (valid) main-thread frame
production that takes a long time and eventually produces updates.

The prior mechanism that would look at the FrameSkippedReason to
identify whether the main thread was active could not detect no-op
main thread animations where the BeginMainFrame exceeded the BeginFrame
deadline.

Bug: b:187392644
Bug: 1166695
Change-Id: Ia3feb109f0ae498dd378cc7c063790433f21e04d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2877048
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#881990}

• cc/BUILD.gn[diff]
• cc/DEPS[diff]
• cc/mojo_embedder/async_layer_tree_frame_sink.cc[diff]
• cc/scheduler/scheduler.cc[diff]
• cc/scheduler/scheduler.h[diff]
• cc/scheduler/scheduler_state_machine.cc[diff]
• cc/scheduler/scheduler_state_machine.h[diff]
• cc/scheduler/scheduler_unittest.cc[diff]
• cc/test/scheduler_test_common.cc[diff]
• cc/test/scheduler_test_common.h[diff]
• cc/trees/proxy_impl.cc[diff]
• cc/trees/single_thread_proxy.cc[diff]
• components/power_scheduler/power_mode.cc[diff]
• components/power_scheduler/power_mode.h[diff]
• components/power_scheduler/power_mode_arbiter.cc[diff]
• components/power_scheduler/power_mode_arbiter.h[diff]
• components/power_scheduler/power_mode_voter.cc[diff]
• components/power_scheduler/power_mode_voter.h[diff]
• content/common/android/cpu_time_metrics.cc[diff]
• tools/metrics/histograms/enums.xml[diff]