Metal: add programmatic command buffer capture

* Update capture method to use latest interface
* Add simple hooks to enable command buffer capture for dm, nanobench, etc.

Change-Id: Ia69f1d10f1f80084342f4f38d12aca92f4dc710c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/447193
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
1 file changed