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

ui/platform_window/x11/x11_window.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_PLATFORM_WINDOW_X11_X11_WINDOW_H_
 #define UI_PLATFORM_WINDOW_X11_X11_WINDOW_H_
 
 #include "ui/events/platform/platform_event_dispatcher.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/x/x11_atom_cache.h"
@@ skipping 48 matching lines @@
   // is the bounds requested using XConfigureWindow, and |confirmed_bounds_| is
   // the bounds the X11 server has set on the window.
   gfx::Rect requested_bounds_;
   gfx::Rect confirmed_bounds_;
 
   bool window_mapped_;
 
   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_