Fix a crash due to reentrancy in the widget while processing a theme changed notification after com…

ui/views/widget/widget.h

Index: ui/views/widget/widget.h
diff --git a/ui/views/widget/widget.h b/ui/views/widget/widget.h
index 33c065643f1cfb6ce82f3dff3b9e4f2ef367b338..505beb1e9e4b3b5d9c4b1fc3091230c61326a26a 100644
--- a/ui/views/widget/widget.h
+++ b/ui/views/widget/widget.h
@@ -992,6 +992,9 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
 
   ScopedObserver<ui::NativeTheme, ui::NativeThemeObserver> observer_manager_;
 
+  // Guard to avoid reentrancy while processing a theme changed message.
+  bool processing_theme_changed_;
+
   DISALLOW_COPY_AND_ASSIGN(Widget);
 };