[Aura] Support always-on-top top level window.

ui/base/x/x11_util.h

 // Copyright (c) 2011 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_BASE_X_X11_UTIL_H_
 #define UI_BASE_X_X11_UTIL_H_
 #pragma once
 
 // This file declares utility functions for X11 (Linux only).
 //
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 };
 // Return the shared memory type of our X connection.
 UI_EXPORT SharedMemorySupport QuerySharedMemorySupport(Display* dpy);
 
 // Return true iff the display supports Xrender
 UI_EXPORT bool QueryRenderSupport(Display* dpy);
 
 // Return the default screen number for the display
 int GetDefaultScreen(Display* display);
 
+// TODO(xiyuan): Fix the stale XCursorCache problem properly.
+// A special cursor that makes GetXCursor below to clear its XCursorCache.
+const int kCursorClearXCursorCache = -1;
+
 // Returns an X11 Cursor, sharable across the process.
 // |cursor_shape| is an X font cursor shape, see XCreateFontCursor().
 UI_EXPORT Cursor GetXCursor(int cursor_shape);
 
 // These functions do not cache their results --------------------------
 
 // Get the X window id for the default root window
 UI_EXPORT XID GetX11RootWindow();
 
 // Returns the user's current desktop.
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 // the process if called. Use SetX11ErrorHandlers() from x11_util_internal.h
 // to set your own error handlers.
 UI_EXPORT void SetDefaultX11ErrorHandlers();
 
 // Return true if a given window is in full-screen mode.
 UI_EXPORT bool IsX11WindowFullScreen(XID window);
 
 }  // namespace ui
 
 #endif  // UI_BASE_X_X11_UTIL_H_