The OpenGL backend on Windows uses D3D11 to implement several interop
extensions.  Without this change, there are compile failures when the
D3D11 backend is disabled.

BUG=angleproject:2276

Change-Id: I69ad524ca84aa0aa399cfc0b601315f3ee65ed16
Reviewed-on: https://chromium-review.googlesource.com/809186Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>