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This CL adds renderer-side event dispatch breakdowns to EventLatency
UKM. These include breakdowns of both main and compositor threads.

These changes to EventLatency UKM has been approved in:
https://docs.google.com/document/d/1SDxdua997Gm7ihJFVjinHz9qTdBzTJrEzC7GEeQHs-8/edit?usp=sharing

Bug: 1054009
Change-Id: Ic5a4f6b06a2e2c29165fd38ee95c0320877608dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612931Reviewed-by: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843278}