Finally we can isolate this method into the D3D-only code, and
remove the interface from ProgramImpl.

BUG=angleproject:1123

Change-Id: I253b15d44d4cf8bbb5dbeaa78bbcc431a5267bf7
Reviewed-on: https://chromium-review.googlesource.com/293765Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>