x11: Use opaque window for dnd without composite.

ui/base/x/x11_util.h

Index: ui/base/x/x11_util.h
diff --git a/ui/base/x/x11_util.h b/ui/base/x/x11_util.h
index 1ca840418089c6c8c994fda03a167b5edfb418c8..3dfdcb1b849dcf378b57a75ddf5f69bac1a38bf6 100644
--- a/ui/base/x/x11_util.h
+++ b/ui/base/x/x11_util.h
@@ -274,6 +274,9 @@ UI_BASE_X_EXPORT WindowManagerName GuessWindowManager();
 // 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.