Modify ImageTransportSurfaceOverlayMac to allow pipelining

gpu/ipc/service/image_transport_surface_overlay_mac.h

Index: gpu/ipc/service/image_transport_surface_overlay_mac.h
diff --git a/gpu/ipc/service/image_transport_surface_overlay_mac.h b/gpu/ipc/service/image_transport_surface_overlay_mac.h
index d8b4fce5690306e65abd8160d9dc42550b1f2984..4c776625eda7a445c27588efcfac6050eb4e74d8 100644
--- a/gpu/ipc/service/image_transport_surface_overlay_mac.h
+++ b/gpu/ipc/service/image_transport_surface_overlay_mac.h
@@ -25,6 +25,10 @@ namespace ui {
 class CALayerTreeCoordinator;
 }
 
+namespace gl {
+class GLFence;
+}
+
 namespace gpu {
 
 class ImageTransportSurfaceOverlayMac : public gl::GLSurface,
@@ -94,6 +98,8 @@ class ImageTransportSurfaceOverlayMac : public gl::GLSurface,
   gfx::Size pixel_size_;
   float scale_factor_;
 
+  std::unique_ptr<gl::GLFence> previous_frame_fence_;
+
   // The renderer ID that all contexts made current to this surface should be
   // targeting.
   GLint gl_renderer_id_;