Move some D3D11-specific stuff into Renderer11, and remove a few
virtual methods that weren't needed to be virtual.

BUG=angleproject:1369

Change-Id: Id37e7271ffc28b089dbea123dca70f38c1a06ffb
Reviewed-on: https://chromium-review.googlesource.com/345913Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>