Fixes WebGL D3D9 conformance failures in:
    context-attributes-alpha-depth-stencil-antialias
    copy-tex-image-2d-formats.html
    texture-attachment-formats.html

BUG=angle:550

Change-Id: I09d42c5e6cf7d880841207627fd20698ed188a6a
Reviewed-on: https://chromium-review.googlesource.com/191935Tested-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>