Add callback to flush for knowing when gpu is finished work.

Bug: skia:8802
Change-Id: I093c2a4e879b635b169a849d9af3e9f7a3d84a88
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207870
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
34 files changed