Removed an assert about addTraceEvent not returning empty handles. 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/1120841 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>