cc: Optimise shared tile handling in eviction tile priority queue.

cc/resources/eviction_tile_priority_queue.h

Index: cc/resources/eviction_tile_priority_queue.h
diff --git a/cc/resources/eviction_tile_priority_queue.h b/cc/resources/eviction_tile_priority_queue.h
index 6f5d96455d7ec7e188e3f2e3cbfde143f581d219..1bf3c73f65e2fc0eecdd3cab0534994b4d0a5e94 100644
--- a/cc/resources/eviction_tile_priority_queue.h
+++ b/cc/resources/eviction_tile_priority_queue.h
@@ -5,6 +5,7 @@
 #ifndef CC_RESOURCES_EVICTION_TILE_PRIORITY_QUEUE_H_
 #define CC_RESOURCES_EVICTION_TILE_PRIORITY_QUEUE_H_
 
+#include <set>
 #include <utility>
 #include <vector>
 
@@ -32,8 +33,8 @@ class CC_EXPORT EvictionTilePriorityQueue {
     scoped_ptr<TilingSetEvictionQueue> active_queue;
     scoped_ptr<TilingSetEvictionQueue> pending_queue;
 
-    // TODO(vmpstr): Investigate removing this.
-    std::vector<Tile*> returned_shared_tiles;
+    // Set of returned tiles (excluding the current one) for DCHECKing.
+    std::set<const Tile*> returned_tiles_for_debug;
   };
 
   EvictionTilePriorityQueue();