Also adds the support code to allow our TestContext to create a Metal
backend.
Bug: skia:
Change-Id: Ia850687019d79b897bb16e2c151f4f8526721ad9
Reviewed-on: https://skia-review.googlesource.com/22644
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
