Texture formats that are internal to ANGLE can now be uniquely identified
using the ANGLEFormat enum.

GetANGLEFormatInfo returns a structure with the DXGI texture formats
associated with the ANGLE format.

For starters, the ANGLEFormat enum is used when building texture info
structures queried from GetTextureFormatInfo.

BUG=angleproject:1244
TEST=angle_end2end_tests

Change-Id: I58a2027a1d9b7a5621f943cc5ceeb88a858009e3
Reviewed-on: https://chromium-review.googlesource.com/328254Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>