perftests: Record trace events to JSON file. 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>