The draw-call path was outputting color unconditionally even though only
depth/stencil was written by the shader, causing flakiness.

Bug: angleproject:3200
Change-Id: I1b28e37160153863e7e6c2875c3002abf4ec176b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1682729Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>