Some apps actually do this. One of them is gfxbench. This helps
performance by dropping the redundant clear calls.

Bug: b/159489614
Change-Id: Ib7958042f081b8fd58c5bc912fbb45bb223aec0f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2255643Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>