Fix NGBox's iterator logic

third_party/WebKit/Source/core/layout/ng/ng_box.h

Index: third_party/WebKit/Source/core/layout/ng/ng_box.h
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_box.h b/third_party/WebKit/Source/core/layout/ng/ng_box.h
index 790d8897b41484b939a6081d12923469a82410c7..0fb7f198a6678d99326c34cf0f1ac67f9c6707eb 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_box.h
+++ b/third_party/WebKit/Source/core/layout/ng/ng_box.h
@@ -22,6 +22,8 @@ class NGPhysicalFragment;
 class CORE_EXPORT NGBox final : public GarbageCollectedFinalized<NGBox> {
  public:
   explicit NGBox(LayoutObject*);
+
+  // TODO(layout-ng): make it private and declare a friend class to use in tests
   explicit NGBox(ComputedStyle*);
 
   // Returns true when done; when this function returns false, it has to be
@@ -32,9 +34,9 @@ class CORE_EXPORT NGBox final : public GarbageCollectedFinalized<NGBox> {
   bool Layout(const NGConstraintSpace*, NGFragment**);
   const ComputedStyle* Style() const;
 
-  NGBox* NextSibling() const;
+  NGBox* NextSibling();
 
-  NGBox* FirstChild() const;
+  NGBox* FirstChild();
 
   void SetNextSibling(NGBox*);
   void SetFirstChild(NGBox*);