Adds a type enum to WindowContext to determine which kind of
GPU context (GrDirectContext or skgpu::Context) we're using.
Bug: skia:12466
Change-Id: I288878740392a43cd9e82c925fbe2c372d140dc5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/454699
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
