Mojo C++ bindings: Add a SINGLE_INTERFACE_WITH_SYNC_REQUESTS mode for MultiplexRouter.

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

 // Copyright 2016 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/sync_handle_registry.h"
 
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/stl_util.h"
 #include "base/threading/thread_local.h"
@@ skipping 89 matching lines @@
   CHECK_EQ(MOJO_RESULT_OK, result);
   wait_set_handle_.reset(Handle(handle));
   CHECK(wait_set_handle_.is_valid());
 
   DCHECK(!g_current_sync_handle_watcher.Pointer()->Get());
   g_current_sync_handle_watcher.Pointer()->Set(this);
 }
 
 SyncHandleRegistry::~SyncHandleRegistry() {
   DCHECK(thread_checker_.CalledOnValidThread());
+
+  // If this breaks, it is likely that the global variable is bulit into and
+  // accessed from multiple modules.
+  CHECK_EQ(this, g_current_sync_handle_watcher.Pointer()->Get());
+
   g_current_sync_handle_watcher.Pointer()->Set(nullptr);
 }
 
 }  // namespace mojo