Make getAdapterIdentifier D3D-only.

We only use this Renderer method in ProgramD3D, so we can hide it
from the GL-side API.

BUG=angle:773

Change-Id: I7a8092a7e3bceb1498f4e4022f84486e0295759d
Reviewed-on: https://chromium-review.googlesource.com/229060
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>