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

cc/picture_layer_tiling.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_PICTURE_LAYER_TILING_H_
 #define CC_PICTURE_LAYER_TILING_H_
 
 #include "base/basictypes.h"
 #include "base/hash_tables.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 126 matching lines @@
       const gfx::Transform& current_screen_transform,
       int current_source_frame_number,
       double current_frame_time,
       bool store_screen_space_quads_on_tiles);
 
   // Copies the src_tree priority into the dst_tree priority for all tiles.
   // The src_tree priority is reset to the lowest priority possible.  This
   // 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;
 
   PictureLayerTiling(float contents_scale);
   Tile* TileAt(int, int) const;
   void CreateTilesFromContentRect(gfx::Rect layer_rect);
   void CreateTile(int i, int j);
 
   PictureLayerTilingClient* client_;
   float contents_scale_;
   gfx::Size layer_bounds_;
   gfx::Rect last_prioritized_rect_;
   // It is not legal to have a NULL tile in the tiles_ map.
   TileMap tiles_;
   TilingData tiling_data_;
   TileResolution resolution_;
   int last_source_frame_number_;
   double last_impl_frame_time_;
 
   friend class Iterator;
 };
 
 }  // namespace cc
 
 #endif  // CC_PICTURE_LAYER_TILING_H_