implement the logic to set tile priorities based on current matrix

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 d1ccc8213afdae1988f8a51ce266077916c5c05e..6519aeb8fecada5ef85fb8ec445ad6732964c153 100644
--- a/cc/picture_layer_tiling.h
+++ b/cc/picture_layer_tiling.h
@@ -86,7 +86,23 @@ class CC_EXPORT PictureLayerTiling {
 
   void Reset() { return tiles_.clear(); }
 
+  void UpdateTilePriorities(const gfx::Size& view_port,
+                            float layer_content_scale,
+                            const gfx::Transform& last_transform,
+                            const gfx::Transform& current_transform,
+                            double time_delta);
+
  protected:
+  // TODO(qinmin): modify ui/range/Range.h to support template so that we
+  // don't need to define this.
+  struct Range {
+    Range(double start, double end) : start_(start), end_(end) {}
+    Range Intersects(const Range& other);
+    bool IsEmpty();
+    double start_;
+    double end_;
+  };
+
   typedef std::pair<int, int> TileMapKey;
   typedef base::hash_map<TileMapKey, scoped_refptr<Tile> > TileMap;
 
@@ -94,6 +110,20 @@ class CC_EXPORT PictureLayerTiling {
   Tile* TileAt(int, int) const;
   void CreateTile(int i, int j);
 
+  // Calculate the time for the |current_bounds| to intersect with the
+  // |target_bounds| given its previous location and time delta.
+  // This function should work for both scaling and scrolling case.
+  double TimeForBoundsToIntersect(gfx::Rect previous_bounds,
+                                  gfx::Rect current_bounds,
+                                  double time_delta,
+                                  gfx::Rect target_bounds);
+
+  // Calculate a time range that |value| will be larger than |threshold|
+  // given the velocity of its change.
+  Range TimeRangeValueLargerThanThreshold(int value,
+                                          int threshold,
+                                          double velocity);
+
   PictureLayerTilingClient* client_;
   float contents_scale_;
   gfx::Size layer_bounds_;
@@ -101,6 +131,7 @@ class CC_EXPORT PictureLayerTiling {
   TilingData tiling_data_;
 
   friend class Iterator;
+  friend class PictureLayerTilingTest;
 };
 
 }  // namespace cc