changing std::vector to std::set for all_tiles_

cc/tile_manager.h

Index: cc/tile_manager.h
diff --git a/cc/tile_manager.h b/cc/tile_manager.h
index 6e42f75bee75d49f48d65720cabd7eb8a46e4a37..5a30e4cbba13e51ba4cf796d0849c8f037de8a82 100644
--- a/cc/tile_manager.h
+++ b/cc/tile_manager.h
@@ -7,6 +7,7 @@
 
 #include <list>
 #include <queue>
+#include <set>
 #include <vector>
 
 #include "base/hash_tables.h"
@@ -192,7 +193,8 @@ class CC_EXPORT TileManager {
   GlobalStateThatImpactsTilePriority global_state_;
 
   typedef std::vector<Tile*> TileVector;
-  TileVector all_tiles_;
+  typedef std::set<Tile*> TileSet;
+  TileSet all_tiles_;
   TileVector live_or_allocated_tiles_;
   TileVector tiles_that_need_to_be_rasterized_;