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This CL adds two metrics for display compositing.
GpuBlockedBetweenSwapsUs: Measures amount of time Gpu Main thread was
blocked during executing tasks for current frame.

SwapTimeUs: Measures time of glSwapBuffers call to evaluate if the
most blocking happens inside swap buffers or at different time

Change-Id: I089c5dbadabb53b128fcb043d9000521a873fefc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876911
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711398}