This is needed to enable removing GetDXGIFormatInfo calls, which are
difficult to use correctly due to a texture format being associated
with multiple DXGI formats. This is done in preparation of changing
some of the DXGI formats associated with integer textures.

BUG=angleproject:1244
TEST=angle_end2end_tests,
     dEQP-GLES3.functional.fbo.* (no regressions)

Change-Id: I820b8331e6f9aacfe9979b13118371687c5fca51
Reviewed-on: https://chromium-review.googlesource.com/329075Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>