This legacy struct duplicates some methods in the gl::State. We
can restrict its use to D3D and on newer back-ends use the State
directly.

BUG=angleproject:930

Change-Id: I2c298e76b072ee73f2b3e17f6696693031ce1f91
Reviewed-on: https://chromium-review.googlesource.com/258070Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>