This allows us to delete some duplicated code in the D3D11-side.

BUG=angleproject:1389
BUG=angleproject:1459

Change-Id: Ifdcfcd4a56e06ff2ae8f5ca0bda72281d52c2964
Reviewed-on: https://chromium-review.googlesource.com/392208
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>