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>