On some configs, this would use an RGBA4 16-bit backbuffer, which is
not enough resolution for the test. Fix this by requiring 32-bit
backbuffers. We might also want to figure out if we want to default
to 16-bit on Windows 8 desktop, which seems less than ideal.

BUG=angleproject:959

Change-Id: I53b3a9ef6cbecf76c7c764623e5eb1f454a6b72d
Reviewed-on: https://chromium-review.googlesource.com/283773Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Austin Kinross <austin.kinross@gmail.com>
Reviewed-by: Cooper Partin <coopp@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>