This is the initial draft of a spec that extends
EGL_EXT_platform_base to allow choosing a particular ANGLE D3D
backend.

Change-Id: I1d80fa8b3c4a59fe998f6d15aa0f393bdb9d26c1
Reviewed-on: https://chromium-review.googlesource.com/184975Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Tested-by: Scott Graham <scottmg@chromium.org>