[scheduler] Fix task flow annotations in TaskScheduler

Pass event names to the TaskAnnotator from TaskScheduler, which ensures
that we get correct flow events.

Also ensure that we have a correct scope for these trace events.

R=gab@chromium.org,fdoray@chromium.org
BUG=932167

Change-Id: If9f1abcd43fa86d41d2da90d448e507cbd10b838
Reviewed-on: https://chromium-review.googlesource.com/c/1461123
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632951}
4 files changed