mojo: Fix running view-manager related apptests on X11.

ui/platform_window/x11/x11_window.h

Index: ui/platform_window/x11/x11_window.h
diff --git a/ui/platform_window/x11/x11_window.h b/ui/platform_window/x11/x11_window.h
index b964b89476c2701349b6223654a7d542c63c86bc..0b6b9cb32f3cbb37d68c8f82b88e7b34f25dae51 100644
--- a/ui/platform_window/x11/x11_window.h
+++ b/ui/platform_window/x11/x11_window.h
@@ -66,6 +66,18 @@ class X11_WINDOW_EXPORT X11Window : public PlatformWindow,
   DISALLOW_COPY_AND_ASSIGN(X11Window);
 };
 
+namespace test {
+
+// Sets the value of the |override_redirect| flag when creating an X11 window.
+// It is necessary to set this flag on for various tests, otherwise the call to
+// X11Window::Show() blocks because it never receives the MapNotify event. It is
+// unclear why this is necessary, but might be related to calls to
+// XInitThreads().
+X11_WINDOW_EXPORT void SetUseOverrideRedirectWindowByDefault(
+    bool override_redirect);
+
+}  // namespace test
+
 }  // namespace ui
 
 #endif  // UI_PLATFORM_WINDOW_X11_X11_WINDOW_H_