The NULL backend exposes the CHROMIUM_copy_texture extension but would hit
UNIMPLEMENTEDs when trying to call these functions.

BUG=945596

Change-Id: Ia845e48b21e6004946edb47b6f4f0573df1f70a4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1545525Reviewed-by: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>