DX9 emulates GL_R8_EXT using BGR8.  When we made the switch to using
GL_R8_EXT for YUV decoding, we decreased performance on DX9 machines.
DX9 supports luminance, which we fall back to when R8 isn't available.

BUG=438736

Change-Id: Iaa8d0518d481927d66130ea1bf3a95a750c5a761
Reviewed-on: https://chromium-review.googlesource.com/255662Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>