Add ReadPixels Perf Test Trying to compare the Metal backend to the OpenGL backend and it's useful to have a perf test. Bug: angleproject:7117 Change-Id: I5a7518fdd749841c3dffb1b53a9c4bfb88813c3c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3906338 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Gregg Tavares <gman@chromium.org>