When flushing a surface with semaphores, have last semaphore op call glFlush

Bug: skia:6770
Change-Id: Ia321b3826da87bd0d25ca2b13b7360baa8caf597
Reviewed-on: https://skia-review.googlesource.com/20453
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
4 files changed