Update the sk_app WindowContext to hold a GrDirectContext

This CL is working towards replacing GrContext with the GrDirectContext/
GrRecordingContext pair.

Change-Id: Id4488e1280d76a16c37d58bd8d29fb7f8dde6b1d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301940
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
13 files changed