Fix crash/leak issue in native_view_host_gtk.cc.

views/widget/widget_gtk.cc

Index: views/widget/widget_gtk.cc
diff --git a/views/widget/widget_gtk.cc b/views/widget/widget_gtk.cc
index 8274e2d5445ce69f10240f06890f79ff3c7d6abc..f42f093dfa8b5d6b961b1a6a8eb41382efebadce 100644
--- a/views/widget/widget_gtk.cc
+++ b/views/widget/widget_gtk.cc
@@ -112,6 +112,7 @@ WidgetGtk::WidgetGtk(Type type)
 }
 
 WidgetGtk::~WidgetGtk() {
+  DCHECK(delete_on_destroy_ || widget_ == NULL);
   if (type_ != TYPE_CHILD)
     ActiveWindowWatcherX::RemoveObserver(this);
   MessageLoopForUI::current()->RemoveObserver(this);
@@ -444,8 +445,10 @@ void WidgetGtk::Close() {
 }
 
 void WidgetGtk::CloseNow() {
-  if (widget_)
+  if (widget_) {
     gtk_widget_destroy(widget_);
+    widget_ = NULL;
+  }
 }
 
 void WidgetGtk::Show() {