Includes some stats counter gathering and a few related refactors and
cleanups. Also includes a new overlay widget.

Bug: angleproject:5736
Change-Id: Ida8d2cd815c5b598c6a442dd9bbfdf51e9c05180
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2785431
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>