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We don't need task timing on the UI thread, so this patch arranges for
time only to be sampled if we have an observer that needs it.

Bug: 897751
Change-Id: I5915efdb903813a574ca05cee316cb30fa75b39c
Reviewed-on: https://chromium-review.googlesource.com/c/1297425
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603059}