Add comments about performance of transparent windows and layers

ui/aura/window.h

Index: ui/aura/window.h
diff --git a/ui/aura/window.h b/ui/aura/window.h
index 01c6c96870d4a31c982653e0fd383b9a48d6d88d..313ea8883beab7c5db3daa489a874c1afe82bd3c 100644
--- a/ui/aura/window.h
+++ b/ui/aura/window.h
@@ -108,6 +108,10 @@ class AURA_EXPORT Window : public ui::LayerDelegate,
   void SetTitle(const base::string16& title);
 
   bool transparent() const { return transparent_; }
+
+  // Note: Setting a window transparent has significant performance impact,
+  // especially on low-end Chrome OS devices. Please ensure you are not
+  // adding unnecessary overdraw. When in doubt, talk to the graphics team.
   void SetTransparent(bool transparent);
 
   // See description in Layer::SetFillsBoundsCompletely.
@@ -473,7 +477,7 @@ class AURA_EXPORT Window : public ui::LayerDelegate,
 
   int id_;
 
-  // Whether layer is initialized as non-opaque.
+  // Whether layer is initialized as non-opaque. Defaults to false.
   bool transparent_;
 
   std::unique_ptr<LayoutManager> layout_manager_;