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 2c5ba165dd74f95145584a07675811d383f3064d..5931af7e59e2a0f25a78cc5627e08ad2292fae4a 100644
--- a/ui/views/widget/widget.h
+++ b/ui/views/widget/widget.h
@@ -979,6 +979,9 @@ 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);
 };