x11: Use opaque window for dnd without composite.

ui/base/x/x11_util.cc

Index: ui/base/x/x11_util.cc
diff --git a/ui/base/x/x11_util.cc b/ui/base/x/x11_util.cc
index dbb489fa84c800f3d2738e22514e997be6d4c666..bee09b5aa2d950fc9d5c10be1fcfdccdd337cebb 100644
--- a/ui/base/x/x11_util.cc
+++ b/ui/base/x/x11_util.cc
@@ -1185,6 +1185,12 @@ std::string GuessWindowManagerName() {
   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);
 }