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Previously, we have extensive metric coverage and understanding of types of
work being run on the renderer main thread. Our understanding of work being
done on other renderer threads, however, is very limited. This patch:

1. Moved TaskDurationMetricReporter to util/ so that it can be shared by
child/;
2. Added OnTaskCompleted API in WorkerScheduler and implemented it in
WorkerSchedulerImpl and CompositorScheduler;
3. Report per-thread task duration for main thread, worker thread and
compositor thread.

BUG=788744

Change-Id: Ida597ccb90ff42129fd786f95f0d9810a5d5b745
Reviewed-on: https://chromium-review.googlesource.com/795014Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: lpy <lpy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520368}