Adds MetalWindowContext_ios and hooks it up to Window_ios.
Also includes some minor clean up in other iOS code.
Bug: skia:8737
Change-Id: I2e8a0c755310fbc4ed534f975815c60f8eca130b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/238438
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
