cc: Don't discard tiles unless memory is low

cc/picture_layer_tiling.h

Index: cc/picture_layer_tiling.h
diff --git a/cc/picture_layer_tiling.h b/cc/picture_layer_tiling.h
index e8e3b44222dbd82ecc4929b026cfd0fa2d5014aa..85e502555b9a75f704f4f3ce111a3164695a2a87 100644
--- a/cc/picture_layer_tiling.h
+++ b/cc/picture_layer_tiling.h
@@ -36,13 +36,36 @@ class PictureLayerTilingClient {
 
 class TileHandle {
  public:
-  TileHandle(scoped_refptr<Tile> tile);
-  Tile* tile() const { return tile_.get(); }
+  TileHandle(std::pair<int, int> key, scoped_refptr<Tile> tile);
+  TileHandle(const TileHandle& tile_handle);
+  ~TileHandle();
+
   void RegisterWithTileManager();
   void UnregisterFromTileManager();
+
+  void AddToLiveTileList(PictureLayerTiling* tiling);
+  void RemoveFromLiveTileList();
+
+  std::pair<int, int> key() const { return key_; }
+  Tile* tile() const { return tile_.get(); }
+  bool should_be_live() const { return should_be_live_; }
+  void set_should_be_live(bool should_be_live) {
+    should_be_live_ = should_be_live;
+  }
+
  private:
+  std::pair<int, int> key_;
   scoped_refptr<Tile> tile_;
   scoped_refptr<ManagedTileState> managed_tile_state_;
+
+  // Transiently set in UpdateTilePriorities to track tiles that
+  // are actively desired to be painted.
+  bool should_be_live_;
+
+  // tiling_ is non-NULL only if this is in a tiling's live_tile_list_, in
+  // which case tiling_live_tile_list_iterator_ valid.
+  PictureLayerTiling* tiling_;
+  std::list<TileHandle*>::iterator tiling_live_tile_list_iterator_;
 };
 
 class CC_EXPORT PictureLayerTiling {
@@ -167,6 +190,7 @@ class CC_EXPORT PictureLayerTiling {
  protected:
   typedef std::pair<int, int> TileMapKey;
   typedef base::hash_map<TileMapKey, TileHandle> TileMap;
+  typedef std::list<TileHandle*> TileList;
 
   PictureLayerTiling(float contents_scale);
   Tile* TileAt(int, int) const;
@@ -179,11 +203,13 @@ class CC_EXPORT PictureLayerTiling {
   gfx::Rect last_prioritized_rect_;
   // It is not legal to have a NULL tile in the tiles_ map.
   TileMap tiles_;
+  TileList live_tiles_;
   TilingData tiling_data_;
   TileResolution resolution_;
   int last_source_frame_number_;
   double last_impl_frame_time_;
 
+  friend class TileHandle;
   friend class Iterator;
 };