This completes the initial refactor for the D3D11 back-end.

Bug: angleproject:2738
Change-Id: I6bc59d6a1a724b3c64d6cd904e6748c2acf8f67d
Reviewed-on: https://chromium-review.googlesource.com/1151452
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>