This small helper routine will be used when doing masked clears.

Bug: angleproject:2455
Change-Id: Id309b48f2bc5958abb1276d140c9a3599fb7d2b9
Reviewed-on: https://chromium-review.googlesource.com/1024825Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>