Support B8G8R8A8_UNORM_SRGB as D3D11 texture format.

BUG=angleproject:2172

Change-Id: I2f15a55123601735d7fc6caa40bb056243a91b44
Reviewed-on: https://chromium-review.googlesource.com/758600Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>