Add a convenience to downcast contexts into GrDirectContext

This pattern of needing a safe downcast will continue to grow as we
add more explicit use of GrDirectContext and it's causing long ternary
operations that span multiple lines.

Change-Id: I9e2ebe5156e4245524a52d7c92ed3a8509e53151
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/300901
Commit-Queue: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
Auto-Submit: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
6 files changed