cc: Simplify the logic for deciding to update tile priorities.

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 4c4bb147111abcaaec55ca20e340f70e7bad07d1..22580bb37dced6168b3a759d31aa171e5d3c1a24 100644
--- a/cc/picture_layer_tiling.h
+++ b/cc/picture_layer_tiling.h
@@ -144,6 +144,21 @@ class CC_EXPORT PictureLayerTiling {
   // also updates the pile on each tile to be the current client's pile.
   void DidBecomeActive();
 
+  bool needs_update_tile_priorities(int current_source_frame_number,
+                                    double current_frame_time) const {
+    // In pending tree, UpdateTilePriorities is always called with
+    // calcDrawProperties. We want to update tile priorities:
+    // - Immediately after a commit (first_update_in_new_source_frame).
+    // - On animation ticks after the first frame in the tree
+    //   (first_update_in_new_impl_frame).
+    // In active tree, UpdateTilePriorities is only called during draw. We
+    // want to update priorities:
+    // - Only if properties were not already computed by the pending tree
+    //   and activated for the frame (first_update_in_new_impl_frame).
+    return current_source_frame_number != last_source_frame_number_ ||
+        current_frame_time != last_impl_frame_time_;
+  }
+
  protected:
   typedef std::pair<int, int> TileMapKey;
   typedef base::hash_map<TileMapKey, scoped_refptr<Tile> > TileMap;