Add test for flushing empty surface with semaphores

Bug: skia:
Change-Id: I8d7dcb29c5b4c460aa5137842caf6563448ba5d3
Reviewed-on: https://skia-review.googlesource.com/25181
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
7 files changed