cc: Clean up tilings in UpdateTiles as well as in AppendQuads.

cc/layers/picture_layer_impl.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_LAYERS_PICTURE_LAYER_IMPL_H_
 #define CC_LAYERS_PICTURE_LAYER_IMPL_H_
 
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 97 matching lines @@
   using TileRequirementCheck = bool (PictureLayerTiling::*)(const Tile*) const;
 
   PictureLayerImpl(LayerTreeImpl* tree_impl, int id, bool is_mask);
   PictureLayerTiling* AddTiling(float contents_scale);
   void RemoveAllTilings();
   void AddTilingsForRasterScale();
   bool UpdateTilePriorities(const Occlusion& occlusion_in_content_space);
   virtual bool ShouldAdjustRasterScale() const;
   virtual void RecalculateRasterScales();
   void CleanUpTilingsOnActiveLayer(
-      std::vector<PictureLayerTiling*> used_tilings);
+      const std::vector<PictureLayerTiling*>& used_tilings);
   float MinimumContentsScale() const;
   float MaximumContentsScale() const;
   void ResetRasterScale();
   void UpdateViewportRectForTilePriorityInContentSpace();
   PictureLayerImpl* GetRecycledTwinLayer() const;
 
   void SanityCheckTilingState() const;
   // Checks if all tiles required for a certain action (e.g. activation) are
   // ready to draw.  is_tile_required_callback gets called on all candidate
   // tiles and returns true if the tile is required for the action.
@@ skipping 36 matching lines @@
 
   // Any draw properties derived from |transform|, |viewport|, and |clip|
   // parameters in LayerTreeHostImpl::SetExternalDrawConstraints are not valid
   // for prioritizing tiles during resourceless software draws. This is because
   // resourceless software draws can have wildly different transforms/viewports
   // from regular draws. Save a copy of the required draw properties of the last
   // frame that has a valid viewport for prioritizing tiles.
   gfx::Rect visible_rect_for_tile_priority_;
   gfx::Rect viewport_rect_for_tile_priority_in_content_space_;
 
+  // List of tilings that were used last time we appended quads. This can be
+  // used as an optimization not to remove tilings if they are still being
+  // drawn. Note that accessing this vector should only be done in the context
+  // of comparing pointers, since objects pointed to are not guaranteed to
+  // exist.
+  std::vector<PictureLayerTiling*> last_append_quads_tilings_;
+
   DISALLOW_COPY_AND_ASSIGN(PictureLayerImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_PICTURE_LAYER_IMPL_H_