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>
diff --git a/include/gpu/gl/GrGLInterface.h b/include/gpu/gl/GrGLInterface.h
index 1088047..56e655d 100644
--- a/include/gpu/gl/GrGLInterface.h
+++ b/include/gpu/gl/GrGLInterface.h
@@ -456,6 +456,7 @@
 
         /* ARB_sync */
         GrGLFunction<GrGLFenceSyncProc> fFenceSync;
+        GrGLFunction<GrGLIsSyncProc> fIsSync;
         GrGLFunction<GrGLClientWaitSyncProc> fClientWaitSync;
         GrGLFunction<GrGLWaitSyncProc> fWaitSync;
         GrGLFunction<GrGLDeleteSyncProc> fDeleteSync;