Fix an ODR violation for ModalDialogDelegate found by ASan

ui/views/widget/widget_unittest.cc

Index: ui/views/widget/widget_unittest.cc
diff --git a/ui/views/widget/widget_unittest.cc b/ui/views/widget/widget_unittest.cc
index 41d666e12c110ad307a69f760350f4066482b8cf..95fc77e0acd3fda26940cc855991f4f6d59d608a 100644
--- a/ui/views/widget/widget_unittest.cc
+++ b/ui/views/widget/widget_unittest.cc
@@ -3636,6 +3636,8 @@ TEST_F(WidgetTest, WidgetRemovalsObserverCalledWhenMovingBetweenWidgets) {
 
 #if defined(OS_WIN)
 
+namespace {
+
 // Provides functionality to create a window modal dialog.
 class ModalDialogDelegate : public DialogDelegateView {
 public:
@@ -3651,6 +3653,8 @@ private:
   DISALLOW_COPY_AND_ASSIGN(ModalDialogDelegate);
 };
 
+}  // namespace
+
 // Tests the case where an intervening owner popup window is destroyed out from
 // under the currently active modal top-level window. In this instance, the
 // remaining top-level windows should be re-enabled.