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

cc/resources/raster_tile_priority_queue_required.cc

Index: cc/resources/raster_tile_priority_queue_required.cc
diff --git a/cc/resources/raster_tile_priority_queue_required.cc b/cc/resources/raster_tile_priority_queue_required.cc
index 41c25dd255d22a237f052435506f6d32bb28ccd5..833ffdafaa816329fe62821499fb0628a828e6c5 100644
--- a/cc/resources/raster_tile_priority_queue_required.cc
+++ b/cc/resources/raster_tile_priority_queue_required.cc
@@ -64,7 +64,7 @@ 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();
 }