Add API for flushing surfaces with gpu semaphores

BUG=skia:

Change-Id: Ia4bfef784cd5f2516ceccafce958be18a86f91d1
Reviewed-on: https://skia-review.googlesource.com/11488
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Forrest Reiling <freiling@google.com>
32 files changed