Perf Tests: Trace multiple threads. This uses a little helper list to map thread IDs into simple serials that we output as 'tid' in our trace JSON file. We can use this to analyze the behaviour of the command processor thread. Bug: b/172704839 Change-Id: Ic1b5f3ec8427cb310833d1f589503daec4812681 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2524542 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>