GetAs and GetImplAs are template helpers that can replace all of our
custom "makeTextureD3D", etc methods. This will help save code across
different back-ends.

Change-Id: Ib3215c005bfac5a819c5d8f7d60a73a725241332
Reviewed-on: https://chromium-review.googlesource.com/245390Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>