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The PipelineReporter trace-events can be a bit confusing sometimes
because some child-events seem to not be parented correctly. There seems
to be at least one issue with the trace-viewer: if a parent-event has
only a single child-event, and both have the same start/end timestamps,
then the parenting seems to break.

So change how the trace-events are generated to account for this.
Notable changes:
 . All trace-events are generated at the end of the frame. This makes it
   possible to not report the child-events if there's only one child
   event for a parent. This also moves all the trace-related code into
   one place, clearing up the other code.
 . The 'is single threaded' information in the traces is not really all
   that useful, since that information is already available in the trace
   viewer (only Browser is single-threaded). So remove that information
   from the trace (and the associated plumbing), and instead include the
   BeginFrameId.

BUG=none

Change-Id: I319fb764bf24acba6ed3f8427bb0d0a79a891613
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125822
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Behdad Bakhshinategh <behdadb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754795}