Make surface GetFormat pure virtual, add missing overrides.

gpu/ipc/service/image_transport_surface_overlay_mac.mm

Index: gpu/ipc/service/image_transport_surface_overlay_mac.mm
diff --git a/gpu/ipc/service/image_transport_surface_overlay_mac.mm b/gpu/ipc/service/image_transport_surface_overlay_mac.mm
index 85b432909e0733c7728185c40441fa9bf410b1e8..22e69682310bdeb06a13b9e273c30fabe76fe22c 100644
--- a/gpu/ipc/service/image_transport_surface_overlay_mac.mm
+++ b/gpu/ipc/service/image_transport_surface_overlay_mac.mm
@@ -304,6 +304,10 @@ void* ImageTransportSurfaceOverlayMac::GetHandle() {
   return nullptr;
 }
 
+GLSurfaceFormat ImageTransportSurfaceOverlayMac::GetFormat() {
+  return GLSurfaceFormat();
+}
+
 bool ImageTransportSurfaceOverlayMac::OnMakeCurrent(gl::GLContext* context) {
   // Ensure that the context is on the appropriate GL renderer. The GL renderer
   // will generally only change when the GPU changes.