cc: Separate the priority from the tile and put in new PrioritizedTile

cc/resources/raster_tile_priority_queue_required.cc

 // Copyright 2015 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.
 
 #include "cc/resources/raster_tile_priority_queue_required.h"
 
 #include "cc/resources/tiling_set_raster_queue_required.h"
 
 namespace cc {
 
@@ skipping 52 matching lines @@
     const std::vector<PictureLayerImpl*>& active_layers,
     const std::vector<PictureLayerImpl*>& pending_layers) {
   AppendTilingSetRequiredQueues(active_layers, &tiling_set_queues_);
   AppendTilingSetRequiredQueues(pending_layers, &tiling_set_queues_);
 }
 
 bool RasterTilePriorityQueueRequired::IsEmpty() const {
   return tiling_set_queues_.empty();
 }
 
-Tile* RasterTilePriorityQueueRequired::Top() {
+const PrioritizedTile& RasterTilePriorityQueueRequired::Top() const {
   DCHECK(!IsEmpty());
   return tiling_set_queues_.back()->Top();
 }
 
 void RasterTilePriorityQueueRequired::Pop() {
   DCHECK(!IsEmpty());
   tiling_set_queues_.back()->Pop();
   if (tiling_set_queues_.back()->IsEmpty())
     tiling_set_queues_.pop_back();
 }
 
 }  // namespace cc