[views] Changes iteration over |children_| to use range-based for loops (reland)

ui/views/view.h

Index: ui/views/view.h
diff --git a/ui/views/view.h b/ui/views/view.h
index a6811afca8a5c084b0d81d5ab375e483cc46c006..5c88e2c98852c1d693499e330ba4a4f9243fc89c 100644
--- a/ui/views/view.h
+++ b/ui/views/view.h
@@ -84,6 +84,10 @@ class PostEventDispatchHandler;
 class RootView;
 }
 
+namespace {
+class ScopedChildrenLock;
+}

[sadrul, 2016/12/24 00:13:11]

Oh, missed this, sorry. This should not be in anonymous namespace. This can move
into the internal namespace.

+
 /////////////////////////////////////////////////////////////////////////////
 //
 // View class
@@ -1230,6 +1234,7 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   friend class internal::PostEventDispatchHandler;
   friend class internal::RootView;
   friend class FocusManager;
+  friend class ScopedChildrenLock;
   friend class ViewLayerTest;
   friend class Widget;
 
@@ -1466,6 +1471,12 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   // This view's children.
   Views children_;
 
+#if DCHECK_IS_ON()
+  // True while iterating over |children_|. Used to detect and DCHECK when
+  // |children_| is mutated during iteration.
+  mutable bool iterating_;
+#endif
+
   // Size and disposition ------------------------------------------------------
 
   // This View's bounds in the parent coordinate system.