Factor out ContextType from GrContextFactory.

This solves a TODO and helps disentangle Graphite from Ganesh's
GrContextFactory. (I bumped into this while trying to remove
GrBackendApi::kDawn.)

Change-Id: Ie2529c2b38fb9f0a188de32fbf33600899e90053
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/746788
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
42 files changed