Fixes the behavior of clearWithDraw to keep the alpha channel if
we should not be clearing it.

Bug: angleproject:2667
Change-Id: Id49c1d2ca30ecb5b5bdd8abe00825a6210cfacf6
Reviewed-on: https://chromium-review.googlesource.com/1106052Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>