Allows creation of pbuffers from D3D texture objects.

BUG=540829
BUG=angleproject:1144

Change-Id: If8ea717ef011608cd01357c217837133d726d3ea
Reviewed-on: https://chromium-review.googlesource.com/316804
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>