Make sure channel-associated interface pointers are always safe to call.

mojo/public/cpp/bindings/lib/associated_interface_ptr.cc

+// Copyright 2017 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.
+
+#include "mojo/public/cpp/bindings/associated_interface_ptr.h"
+
+namespace mojo {
+
+void GetIsolatedInterface(ScopedInterfaceEndpointHandle handle) {
+  MessagePipe pipe;
+  scoped_refptr<internal::MultiplexRouter> router =
+      new internal::MultiplexRouter(std::move(pipe.handle0),
+                                    internal::MultiplexRouter::MULTI_INTERFACE,
+                                    false, base::ThreadTaskRunnerHandle::Get());
+  router->AssociateInterface(std::move(handle));
+}
+
+}  // namespace mojo