Add comments about performance of transparent windows and layers

ui/compositor/layer.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_COMPOSITOR_LAYER_H_
 #define UI_COMPOSITOR_LAYER_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 253 matching lines @@
   static void ConvertPointToLayer(const Layer* source,
                                   const Layer* target,
                                   gfx::Point* point);
 
   // Converts a transform to be relative to the given |ancestor|. Returns
   // whether success (that is, whether the given ancestor was really an
   // ancestor of this layer).
   bool GetTargetTransformRelativeTo(const Layer* ancestor,
                                     gfx::Transform* transform) const;
 
-  // See description in View for details
+  // Note: Setting a layer non-opaque 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 SetFillsBoundsOpaquely(bool fills_bounds_opaquely);
   bool fills_bounds_opaquely() const { return fills_bounds_opaquely_; }
 
   // Set to true if this layer always paints completely within its bounds. If so
   // we can omit an unnecessary clear, even if the layer is transparent.
   void SetFillsBoundsCompletely(bool fills_bounds_completely);
 
   const std::string& name() const { return name_; }
   void set_name(const std::string& name) { name_ = name; }
 
@@ skipping 175 matching lines @@
 
   // 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_;
 
   // Visibility of this layer. See SetVisible/IsDrawn for more details.
   bool visible_;
 
+  // See SetFillsBoundsOpaquely(). Defaults to true.
   bool fills_bounds_opaquely_;
+
   bool fills_bounds_completely_;
 
   // Union of damaged rects, in layer space, that SetNeedsDisplayRect should
   // be called on.
   cc::Region damaged_region_;
 
   // Union of damaged rects, in layer space, to be used when compositor is ready
   // to paint the content.
   cc::Region paint_region_;
 
@@ skipping 60 matching lines @@
   // The size of the frame or texture in DIP, set when SetShowDelegatedContent
   // or SetTextureMailbox was called.
   gfx::Size frame_size_in_dip_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_LAYER_H_