cc: Use reverse spiral iterator in tiling eviction.

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 1ad3afb10d3ceb90ebafb00303c917656c2a8763..40bfe431c1c9097987572c543100c638fd67d038 100644
--- a/cc/layers/picture_layer_impl.h
+++ b/cc/layers/picture_layer_impl.h
@@ -70,8 +70,7 @@ class CC_EXPORT PictureLayerImpl
   class CC_EXPORT LayerEvictionTileIterator {
    public:
     LayerEvictionTileIterator();
-    LayerEvictionTileIterator(PictureLayerImpl* layer,
-                              TreePriority tree_priority);
+    explicit LayerEvictionTileIterator(PictureLayerImpl* layer);
     ~LayerEvictionTileIterator();
 
     Tile* operator*();
@@ -88,7 +87,6 @@ class CC_EXPORT PictureLayerImpl
     size_t CurrentTilingIndex() const;
 
     PictureLayerImpl* layer_;
-    TreePriority tree_priority_;
 
     PictureLayerTiling::EvictionCategory current_category_;
     PictureLayerTilingSet::TilingRangeType current_tiling_range_type_;