This fixes the same issue as https://chromium-review.googlesource.com/328400,
except with NV12 external storage textures. The SRV cache thinks the
texture is still bound, but releasing the keyed mutex has unbound it.

BUG=625900,626524

Change-Id: I991cb3eeaaea0a1c4570b88de2ba873ae868ec2a
Reviewed-on: https://chromium-review.googlesource.com/359430Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: John Bauman <jbauman@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/365160Reviewed-by: John Bauman <jbauman@chromium.org>