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 fc1e581f5e88ba458a270881abd5367ca1b27bb0..14cd6f605fb57bd0d534b586cce5734a149219d1 100644
--- a/cc/resources/picture_layer_tiling_set.h
+++ b/cc/resources/picture_layer_tiling_set.h
@@ -36,6 +36,10 @@ 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,
@@ -108,6 +112,8 @@ 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.