[OT-PW] Add metric to measure main-thread fallback rate

This CL adds a new metric to measure how often paint worklet successfully
paint on the compositor thread vs fall back to the main thread.

Bug: 1002076
Change-Id: I65ed849e1cfb992f24d594b025995aa1e78eb009
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1791437
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698949}
6 files changed