Add default flush calls to gpu backend.

Change-Id: I7cd8ff975502128dbde86397218b7e5c115cdaa6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299141
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
4 files changed