x11: Use opaque window for dnd without composite.

ui/base/x/x11_util.cc

 // 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.
 
 // This file defines utility functions for X11 (Linux only). This code has been
 // ported from XCB since we can't use XCB on Ubuntu while its 32-bit support
 // remains woefully incomplete.
 
 #include "ui/base/x/x11_util.h"
 
@@ skipping 1167 matching lines @@
   return WM_OTHER;
 }
 
 std::string GuessWindowManagerName() {
   std::string name;
   if (GetWindowManagerName(&name))
     return name;
   return "Unknown";
 }
 
+bool IsCompositingManagerPresent() {
+  static bool is_compositing_manager_present =
+      XGetSelectionOwner(gfx::GetXDisplay(), GetAtom("_NET_WM_CM_S0")) != None;
+  return is_compositing_manager_present;
+}
+
 void SetDefaultX11ErrorHandlers() {
   SetX11ErrorHandlers(NULL, NULL);
 }
 
 bool IsX11WindowFullScreen(XID window) {
   // If _NET_WM_STATE_FULLSCREEN is in _NET_SUPPORTED, use the presence or
   // absence of _NET_WM_STATE_FULLSCREEN in _NET_WM_STATE to determine
   // whether we're fullscreen.
   XAtom fullscreen_atom = GetAtom("_NET_WM_STATE_FULLSCREEN");
   if (WmSupportsHint(fullscreen_atom)) {
@@ skipping 279 matching lines @@
   return colormap_;
 }
 
 #endif
 
 // ----------------------------------------------------------------------------
 // End of x11_util_internal.h
 
 
 }  // namespace ui