x11: Use opaque window for dnd without composite.

ui/base/x/x11_util.h

 // Copyright (c) 2012 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_
 
 // This file declares utility functions for X11 (Linux only).
 //
 // These functions do not require the Xlib headers to be included (which is why
@@ skipping 256 matching lines @@
   WM_XMONAD,
 };
 // Attempts to guess the window maager. Returns WM_OTHER or WM_UNNAMED
 // if we can't determine it for one reason or another.
 UI_BASE_X_EXPORT WindowManagerName GuessWindowManager();
 
 // The same as GuessWindowManager(), but returns the raw string.  If we
 // can't determine it, return "Unknown".
 UI_BASE_X_EXPORT std::string GuessWindowManagerName();
 
+// Returns true if a compositing manager is present.
+UI_BASE_X_EXPORT bool IsCompositingManagerPresent();
+
 // Enable the default X error handlers. These will log the error and abort
 // the process if called. Use SetX11ErrorHandlers() from x11_util_internal.h
 // to set your own error handlers.
 UI_BASE_X_EXPORT void SetDefaultX11ErrorHandlers();
 
 // Returns true if a given window is in full-screen mode.
 UI_BASE_X_EXPORT bool IsX11WindowFullScreen(XID window);
 
 // Returns true if the window manager supports the given hint.
 UI_BASE_X_EXPORT bool WmSupportsHint(XAtom atom);
@@ skipping 39 matching lines @@
 namespace test {
 
 // Returns the cached XcursorImage for |cursor|.
 UI_BASE_X_EXPORT const XcursorImage* GetCachedXcursorImage(::Cursor cursor);
 
 }  // namespace test
 
 }  // namespace ui
 
 #endif  // UI_BASE_X_X11_UTIL_H_