Add comments about performance of transparent windows and layers

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index 76451c0a8300f4e3dfbaa84c164dd159696ffb69..6295c41a7cf52796d72e2620d9500c1f44bcf6d0 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -271,7 +271,9 @@ class COMPOSITOR_EXPORT 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_; }
 
@@ -467,7 +469,9 @@ class COMPOSITOR_EXPORT Layer
   // 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