add support for clears to Metal backend

Bug: skia:
Change-Id: I35d9fcf096d078d6dc04093e3591df0ac12079b7
Reviewed-on: https://skia-review.googlesource.com/c/161821
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
15 files changed