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

cc/layers/picture_layer_impl.h

Index: cc/layers/picture_layer_impl.h
diff --git a/cc/layers/picture_layer_impl.h b/cc/layers/picture_layer_impl.h
index 86f142457521e2e49fa77577bdc2291d58be0d13..378908f9424fd9774755dd070898e76e18103ea8 100644
--- a/cc/layers/picture_layer_impl.h
+++ b/cc/layers/picture_layer_impl.h
@@ -115,7 +115,7 @@ class CC_EXPORT PictureLayerImpl
   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();
@@ -172,6 +172,13 @@ class CC_EXPORT PictureLayerImpl
   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);
 };