Add fence support for TransferBuffers

include/gpu/gl/GrGLInterface.h

Index: include/gpu/gl/GrGLInterface.h
diff --git a/include/gpu/gl/GrGLInterface.h b/include/gpu/gl/GrGLInterface.h
index 27184adcb5581ddc5c737e4594bfbbb57574801e..60109ec187708c1ad7a4c785e361d7ca7fd0f886 100644
--- a/include/gpu/gl/GrGLInterface.h
+++ b/include/gpu/gl/GrGLInterface.h
@@ -452,6 +452,11 @@ public:
         // OpenGL 3.1
         GrGLFunction<GrGLTextureBufferProc> fTextureBuffer;
 
+        /* ARB_sync */
+        GrGLFunction<GrGLFenceSyncProc> fFenceSync;
+        GrGLFunction<GrGLClientWaitSyncProc> fClientWaitSync;
+        GrGLFunction<GrGLDeleteSyncProc> fDeleteSync;
+
         /* KHR_debug */
         GrGLFunction<GrGLDebugMessageControlProc> fDebugMessageControl;
         GrGLFunction<GrGLDebugMessageInsertProc> fDebugMessageInsert;