Add Metal context to Viewer.

Bug: skia:8737
Change-Id: I5c4c839bcf39f2cd3a9a32d005bf4bdb7c42e6a5
Reviewed-on: https://skia-review.googlesource.com/c/187925
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
8 files changed