[graphite] Flesh out more of Metal Caps.

Allocates and initializes the caps with GpuFamily and Group info.

Bug: skia:12466
Change-Id: I7abb8b6df2acf2ac70f3fbe56e935f44815776dd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/453796
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
6 files changed