cc: Simplify the logic for deciding to update tile priorities.

cc/layer_tree_impl.h

Index: cc/layer_tree_impl.h
diff --git a/cc/layer_tree_impl.h b/cc/layer_tree_impl.h
index 729c3d452816f9baab5e17e851f0e74394e385b8..8e2ee91c7e97b869541d9f13bdacbd3662b599e2 100644
--- a/cc/layer_tree_impl.h
+++ b/cc/layer_tree_impl.h
@@ -152,6 +152,10 @@ class CC_EXPORT LayerTreeImpl {
   void set_needs_full_tree_sync(bool needs) { needs_full_tree_sync_ = needs; }
   bool needs_full_tree_sync() const { return needs_full_tree_sync_; }
 
+  void set_needs_update_tile_priorities() {
+    needs_update_tile_priorities_ = true;
+  }
+
   void ClearRenderSurfaces();
 
   bool AreVisibleResourcesReady() const;
@@ -219,6 +223,7 @@ protected:
   bool contents_textures_purged_;
   bool viewport_size_invalid_;
   bool needs_update_draw_properties_;
+  bool needs_update_tile_priorities_;
 
   // In impl-side painting mode, this is true when the tree may contain
   // structural differences relative to the active tree.