Add support for ANGLE's Metal backend

- Disables MSL via SPIRV (we don't have the necessary DEPS)
- Adds new context type, configs, etc...
- Minor tweaks to the ANGLE test context code

Bug: angleproject:7155
Bug: skia:13272
Change-Id: I258ed19abba01ad96cfe6fca46b558af2340880e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/534569
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
8 files changed