Fully add back old GrContext flush api.

I left off the optional finished proc on the previous cl :(

Change-Id: I5f540c718bb7e12e7ea74287f1e50e35edd79700
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209417
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Auto-Submit: Greg Daniel <egdaniel@google.com>
3 files changed