cc: Give non-drawing layers that are rasterized a lower priority.

cc/layers/layer_impl.h

 // Copyright 2011 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 CC_LAYERS_LAYER_IMPL_H_
 #define CC_LAYERS_LAYER_IMPL_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 546 matching lines @@
 
   bool has_will_change_transform_hint_ : 1;
   bool needs_push_properties_ : 1;
   bool scrollbars_hidden_ : 1;
 
   // The needs_show_scrollbars_ bit tracks a pending request from Blink to show
   // the overlay scrollbars. It's set on the scroll layer (not the scrollbar
   // layers) and consumed by LayerTreeImpl::PushPropertiesTo during activation.
   bool needs_show_scrollbars_ : 1;
 
+  // This is set for layers that have a property because of which they are not
+  // drawn (singular transforms), but they can become visible soon (the property
+  // is being animated). For this reason, while these layers are not drawn, they
+  // are still rasterized.
   bool raster_even_if_not_in_rsll_ : 1;
 
   DISALLOW_COPY_AND_ASSIGN(LayerImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_IMPL_H_