This allows us to view the timeline of events in the trace event
browser. We can extend this to do GPU timestamp queries and analyze
when work is actually in flight.

The trace is enabled in standalone ANGLE only using the flag
--enable-trace with angle_perftests. You can also optionally specify
the trace output file with --trace-file <blah>. The default file is
ANGLETrace.json.

Bug: angleproject:2781
Change-Id: I871f28545d9bf18220b55aaf69e9554dcb4c834d
Reviewed-on: https://chromium-review.googlesource.com/c/1259763
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>