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Currently, we only support thread instruction delta values on
PHASE_COMPLETE events. Since we'll soon replace complete events
(at least on TrackEvent proto level) with begin/end event pairs, this
patch adds support for emitting absolute thread instruction count
values on begin/end events.

Also ensures that we write sensible timestamps for all metadata events
in TraceLog.

Bug: 925589
Change-Id: Ie01339aefb3d148457f51057e0722c4506dda82f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1729262
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682840}