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>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 7ef9c3a..aad169d 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -280,6 +280,8 @@
   "$_src/gpu/ops/GrRectOpFactory.cpp",
   "$_src/gpu/ops/GrRegionOp.cpp",
   "$_src/gpu/ops/GrRegionOp.h",
+  "$_src/gpu/ops/GrSemaphoreOp.cpp",
+  "$_src/gpu/ops/GrSemaphoreOp.h",
   "$_src/gpu/ops/GrShadowRRectOp.cpp",
   "$_src/gpu/ops/GrShadowRRectOp.h",
   "$_src/gpu/ops/GrStencilAndCoverPathRenderer.cpp",