Fix an ODR violation for ModalDialogDelegate found by ASan

ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc

Index: ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc
diff --git a/ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc b/ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc
index 5865426151cb0fe57f4ae88e7d29d133dcae5295..0020aed01bc97b87d75d2b5ae2e26013ba371b78 100644
--- a/ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc
+++ b/ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc
@@ -509,6 +509,8 @@ TEST_F(DesktopAuraWidgetTest, CloseWidgetDuringMouseReleased) {
   RunCloseWidgetDuringDispatchTest(this, ui::ET_MOUSE_RELEASED);
 }
 
+namespace {
+
 // Provides functionality to create a window modal dialog.
 class ModalDialogDelegate : public DialogDelegateView {
  public:
@@ -522,6 +524,8 @@ class ModalDialogDelegate : public DialogDelegateView {
   DISALLOW_COPY_AND_ASSIGN(ModalDialogDelegate);
 };
 
+}  // namespace
+
 // This test verifies that whether mouse events when a modal dialog is
 // displayed are eaten or recieved by the dialog.
 TEST_F(WidgetTest, WindowMouseModalityTest) {