Add GrSemaphoreOp to support deferred semaphore use

This is a precursor to adding support for external drawable ops.

BUG=skia:

Change-Id: I3e8b06b4cbe2b5728b7911c22ee74a93d0813f98
Reviewed-on: https://skia-review.googlesource.com/9452
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
3 files changed