Fix missing trace_event header. Noticed this while investigating the tracing code. BUG=chromium:678870 Change-Id: I016c8532044b8f105ddda67f7e06c6b5ed9c61fb Reviewed-on: https://chromium-review.googlesource.com/437647 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>