We were not properly setting the masked channels for these APIs.
Since we use a std::vector for tracking the render targets for
a particular ClearBuffer call, we lose the indexing into the
clear parameters. Fix this by storing the information in the
std::vector along with the Render Target.

BUG=angle:702

Change-Id: Ie3b1e870aa04054411c4f155682b86a340ec00cf
Reviewed-on: https://chromium-review.googlesource.com/209103Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>