cc: Rework how picture layer tiling set gets into raster queues.

cc/resources/picture_layer_tiling_set.h

Index: cc/resources/picture_layer_tiling_set.h
diff --git a/cc/resources/picture_layer_tiling_set.h b/cc/resources/picture_layer_tiling_set.h
index 3e31ed9bcfc055f72f4dc70d672631214040ea7a..b9e4bdd009af29e5f1cde128df81de0708286ac2 100644
--- a/cc/resources/picture_layer_tiling_set.h
+++ b/cc/resources/picture_layer_tiling_set.h
@@ -42,6 +42,10 @@ class CC_EXPORT PictureLayerTilingSet {
     size_t start;
     size_t end;
   };
+  struct Pair {
+    PictureLayerTilingSet* active = nullptr;
+    PictureLayerTilingSet* pending = nullptr;
+  };
 
   static scoped_ptr<PictureLayerTilingSet> Create(
       PictureLayerTilingClient* client,
@@ -114,6 +118,8 @@ class CC_EXPORT PictureLayerTilingSet {
 
   void GetAllTilesForTracing(std::set<const Tile*>* tiles) const;
 
+  bool HasValidTilePriorities() const;
+
   // For a given rect, iterates through tiles that can fill it.  If no
   // set of tiles with resources can fill the rect, then it will iterate
   // through null tiles with valid geometry_rect() until the rect is full.