There are several codepaths where this is the expected result:
* Other threads disabling the category in question.
* The TraceBuffer being fill and no new chunks available.
* Perfetto being active as the tracing backend.

In all cases, the single use of a handle to a TraceEvent is
TraceLog::UpdateTraceEventDuration which handles the above cases
correctly.

BUG=844421
Change-Id: Ieaf3aa5c913cee8c51cfea637907d5bc3b560ceb
Reviewed-on: https://chromium-review.googlesource.com/1120841Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>