Generalize layer mirroring for phantom windows

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index 7c7e4e32562c1899c69a438587c10cc6aaeafea6..e882bbb5f42a7f43a6bcf26bb0e89780a5c14576 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -76,6 +76,16 @@ class COMPOSITOR_EXPORT Layer
   explicit Layer(LayerType type);
   ~Layer() override;
 
+  // Note that only solid color and surface content is copied.
+  std::unique_ptr<Layer> Clone() const;
+
+  // Returns a new layer that mirrors this layer and is optionally synchronized
+  // with the bounds thereof. Note that children are not mirrored, and that the
+  // content is only mirrored if painted by a delegate or backed by a surface.
+  std::unique_ptr<Layer> Mirror();
+
+  void set_sync_bounds(bool sync_bounds) { sync_bounds_ = sync_bounds; }
+
   // Retrieves the Layer's compositor. The Layer will walk up its parent chain
   // to locate it. Returns NULL if the Layer is not attached to a compositor.
   Compositor* GetCompositor() {
@@ -303,7 +313,7 @@ class COMPOSITOR_EXPORT Layer
 
   // Sets the layer's fill color.  May only be called for LAYER_SOLID_COLOR.
   void SetColor(SkColor color);
-  SkColor GetTargetColor();
+  SkColor GetTargetColor() const;
   SkColor background_color() const;
 
   // Updates the nine patch layer's image, aperture and border. May only be
@@ -383,6 +393,7 @@ class COMPOSITOR_EXPORT Layer
 
  private:
   friend class LayerOwner;
+  class LayerMirror;
 
   void CollectAnimators(std::vector<scoped_refptr<LayerAnimator> >* animators);
 
@@ -433,6 +444,8 @@ class COMPOSITOR_EXPORT Layer
   void SetCompositorForAnimatorsInTree(Compositor* compositor);
   void ResetCompositorForAnimatorsInTree(Compositor* compositor);
 
+  void OnMirrorDestroyed(LayerMirror* mirror);
+
   const LayerType type_;
 
   Compositor* compositor_;
@@ -442,6 +455,11 @@ class COMPOSITOR_EXPORT Layer
   // This layer's children, in bottom-to-top stacking order.
   std::vector<Layer*> children_;
 
+  std::vector<std::unique_ptr<LayerMirror>> mirrors_;
+
+  // If true, changes to the bounds of this layer are propagated to mirrors.
+  bool sync_bounds_ = false;
+
   gfx::Rect bounds_;
   gfx::Vector2dF subpixel_position_offset_;