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This CL extends fps meter to show the throughput. More specifically,
we show the unified throughput of the main && compositor thread for
the Universal tracker. At this moment, the number will be updated every
5 seconds as long as there is a new throughput reported during the
5-second period.

The result looks like:
https://drive.google.com/file/d/1LUxfbK8IINxLFoPi1FoxI_IvnimrXhoR/view?usp=sharing

Bug: 1064290
Change-Id: I257333c0b580eddc570eeb971d913b93256db008
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135537
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756879}