Since the backbuffer's depth stencil is always emualted, it can support any
depth stencil format.

BUG=angleproject:2692

Change-Id: I29df62bb322c1ac4e9fcd54e4cbc4c219cf12e83
Reviewed-on: https://chromium-review.googlesource.com/1119075
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>