cc: Change TileManager iterators to be queues.

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 3b07c5dfd367897ce1e7b7524846c66c1908072a..7af27f977f60eb48af89e6948a2f081a35b8d968 100644
--- a/cc/layers/picture_layer_impl.h
+++ b/cc/layers/picture_layer_impl.h
@@ -27,6 +27,15 @@ class CC_EXPORT PictureLayerImpl
     : public LayerImpl,
       NON_EXPORTED_BASE(public PictureLayerTilingClient) {
  public:
+  struct CC_EXPORT Pair {
+    Pair();
+    Pair(PictureLayerImpl* active_layer, PictureLayerImpl* pending_layer);
+    ~Pair();
+
+    PictureLayerImpl* active;
+    PictureLayerImpl* pending;
+  };
+
   class CC_EXPORT LayerRasterTileIterator {
    public:
     LayerRasterTileIterator();