This perf test measures the GPU time spent clearing framebuffers.

Bug: angleproject:2361
Change-Id: I5dc2e12c08543330f7e2565596b73e9c2f4e53ef
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1529864
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>