Remove GrGpu::testingOnly_flushGpuAndSync

Submitting without additional flushing and syncing GPU/CPU is achievable
with public APIs. Moreover, this testing-only method bypasses code and
can lead to bugs.

Change-Id: I03bd0fa65e5ba88ba6181521947243ba84ab9696
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369876
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
18 files changed