Migrate flush & misc methods to GrDirectContext

More cut and paste work.

Change-Id: I0dfc822ae168e15c6734a6c079bb930fc0fa9e60
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/325618
Commit-Queue: Adlai Holler <adlai@google.com>
Auto-Submit: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
6 files changed