Combine GPU flush with semaphore API

Bug: skia:
Change-Id: I5f388760d43b19755b2767d95d32065f6d0f926e
Reviewed-on: https://skia-review.googlesource.com/14646
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
7 files changed