cc: Remove LayerImpl::children() calls after tree synchronization

cc/trees/damage_tracker_unittest.cc

Index: cc/trees/damage_tracker_unittest.cc
diff --git a/cc/trees/damage_tracker_unittest.cc b/cc/trees/damage_tracker_unittest.cc
index e276c01f3ed770995d67c2a967e9155a9f769c8c..11190770b81121afd6d1afc8bb97dc6fa94c9408 100644
--- a/cc/trees/damage_tracker_unittest.cc
+++ b/cc/trees/damage_tracker_unittest.cc
@@ -42,13 +42,11 @@ void ExecuteCalculateDrawProperties(LayerImpl* root,
   ASSERT_TRUE(root->render_surface());
 }
 
-void ClearDamageForAllSurfaces(LayerImpl* layer) {
-  if (layer->render_surface())
-    layer->render_surface()->damage_tracker()->DidDrawDamagedArea();
-
-  // Recursively clear damage for any existing surface.
-  for (size_t i = 0; i < layer->children().size(); ++i)
-    ClearDamageForAllSurfaces(layer->children()[i]);
+void ClearDamageForAllSurfaces(LayerImpl* root) {
+  for (auto* layer : *root->layer_tree_impl()) {
+    if (layer->render_surface())
+      layer->render_surface()->damage_tracker()->DidDrawDamagedArea();
+  }
 }
 
 void EmulateDrawingOneFrame(LayerImpl* root) {