Clear all channels to known state before clearing with one masked,
so we know what to expect in the masked one.

BUG=angleproject:2674

Change-Id: Icbe58615c68e2b282a27d8f29c2f2a48fb8b1c3e
Reviewed-on: https://chromium-review.googlesource.com/1212822
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>