Fixes the crash when the transient parent is NULL.

chrome/browser/ui/libgtk2ui/gtk2_util.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_UI_LIBGTK2UI_GTK2_UTIL_H_
 #define CHROME_BROWSER_UI_LIBGTK2UI_GTK2_UTIL_H_
 
 #include <gtk/gtk.h>
 #include <string>
 
@@ skipping 26 matching lines @@
 
 guint GetGdkKeyCodeForAccelerator(const ui::Accelerator& accelerator);
 
 GdkModifierType GetGdkModifierForAccelerator(
     const ui::Accelerator& accelerator);
 
 // Translates event flags into plaform independent event flags.
 int EventFlagsFromGdkState(guint state);
 
 // Sets |dialog| as transient for |parent|, which will keep it on top and center
-// it above |parent|.
+// it above |parent|. Do nothing if |parent| is NULL.
 void SetGtkTransientForAura(GtkWidget* dialog, aura::Window* parent);
 
 // Gets the transient parent aura window for |dialog|.
 aura::Window* GetAuraTransientParent(GtkWidget* dialog);
 
 // Clears the transient parent for |dialog|.
 void ClearAuraTransientParent(GtkWidget* dialog);
 
 }  // namespace libgtk2ui
 
 #endif  // CHROME_BROWSER_UI_LIBGTK2UI_GTK2_UTIL_H_