Add comments about performance of transparent windows and layers

ui/aura/window.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_AURA_WINDOW_H_
 #define UI_AURA_WINDOW_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 90 matching lines @@
   int id() const { return id_; }
   void set_id(int id) { id_ = id; }
 
   const std::string& GetName() const;
   void SetName(const std::string& name);
 
   const base::string16& GetTitle() const;
   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.
   void SetFillsBoundsCompletely(bool fills_bounds);
 
   WindowDelegate* delegate() { return delegate_; }
   const WindowDelegate* delegate() const { return delegate_; }
 
   const gfx::Rect& bounds() const { return bounds_; }
 
@@ skipping 345 matching lines @@
   // Child windows. Topmost is last.
   Windows children_;
 
   // The visibility state of the window as set by Show()/Hide(). This may differ
   // from the visibility of the underlying layer, which may remain visible after
   // the window is hidden (e.g. to animate its disappearance).
   bool visible_;
 
   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_;
   std::unique_ptr<ui::EventTargeter> targeter_;
 
   // Makes the window pass all events through to any windows behind it.
   bool ignore_events_;
 
   // See set_hit_test_bounds_override_inner().
   gfx::Insets hit_test_bounds_override_inner_;
 
   base::ObserverList<WindowObserver, true> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(Window);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_WINDOW_H_