Fix masked ClearBuffer. 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/209103 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>