Generalize layer mirroring for phantom windows

cc/layers/surface_layer.h

Index: cc/layers/surface_layer.h
diff --git a/cc/layers/surface_layer.h b/cc/layers/surface_layer.h
index 06c84c1829b05cc09dbf185baa2fd9aca6c1d7c1..a66257d1bd94f6096dee8d9a9a614f70ad0777c8 100644
--- a/cc/layers/surface_layer.h
+++ b/cc/layers/surface_layer.h
@@ -41,6 +41,13 @@ class CC_EXPORT SurfaceLayer : public Layer {
   void SetLayerTreeHost(LayerTreeHost* host) override;
   void PushPropertiesTo(LayerImpl* layer) override;
 
+  SurfaceId surface_id() const { return surface_id_; }
+  const gfx::Size& surface_size() const { return surface_size_; }
+  float surface_scale() const { return surface_scale_; }
+
+  const SatisfyCallback& satisfy_callback() const { return satisfy_callback_; }
+  const RequireCallback& require_callback() const { return require_callback_; }
+
  protected:
   SurfaceLayer(const SatisfyCallback& satisfy_callback,
                const RequireCallback& require_callback);