Add always-on-top property to app windows

apps/native_app_window.h

 // Copyright 2013 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 APPS_NATIVE_APP_WINDOW_H_
 #define APPS_NATIVE_APP_WINDOW_H_
 
 #include "apps/shell_window.h"
 #include "components/web_modal/web_contents_modal_dialog_host.h"
 #include "ui/base/base_window.h"
@@ skipping 35 matching lines @@
   // Returns the difference between the window bounds (including titlebar and
   // borders) and the content bounds, if any.
   virtual gfx::Insets GetFrameInsets() const = 0;
 
   // Hide or show this window as part of hiding or showing the app.
   // This may have different logic to Hide, Show, and ShowInactive as those are
   // called via the AppWindow javascript API.
   virtual void ShowWithApp() = 0;
   virtual void HideWithApp() = 0;
 
+  // If |always_on_top| is true, the window will stay on top of windows that do
+  // not have this flag enabled.
+  virtual void SetAlwaysOnTop(bool always_on_top) = 0;
+
   virtual ~NativeAppWindow() {}
 };
 
 }  // namespace apps
 
 #endif  // APPS_NATIVE_APP_WINDOW_H_