Add support to gpu command buffers to wrap an external command buffer.

Bug: skia:
Change-Id: Ic679d292f42c61f9f1c36315ae605504a0283306
Reviewed-on: https://skia-review.googlesource.com/c/179521
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>