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This CL make the frame sequence tracker reports an aggregated metric
for both compositor + main thread. At this moment, it reports the worse
throughput between the compositor vs main thread. Unit tests are added.

Bug: 1002994
Change-Id: I777c68ea04929e287a91cf82160818be34e36f45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1801921Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699585}