Fix flaky tests OffscreenCanvasClearRect*

Reimplements flaky ref tests as testharness tests. This approach
avoids being dependent on the compositor's timing.

Bug: 1088007
Change-Id: I726101c87c814a120f7ebc977c5d75951106505d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4347610


Commit-Queue: Justin Novosad <junov@chromium.org>
Reviewed-by: Yi Xu <yiyix@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118917}