Make asDirectContext return a GrDirectContext

Change-Id: I373658d68582adc9728f3a75d84178a365f5b798
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299877
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Adlai Holler <adlai@google.com>
30 files changed