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

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

Index: mojo/public/cpp/bindings/lib/sync_handle_registry.cc
diff --git a/mojo/public/cpp/bindings/lib/sync_handle_registry.cc b/mojo/public/cpp/bindings/lib/sync_handle_registry.cc
index ef236953404b92873a3812f701a51171a822e9da..cc8313717577821d89b56f0d1c5c78feebcafc26 100644
--- a/mojo/public/cpp/bindings/lib/sync_handle_registry.cc
+++ b/mojo/public/cpp/bindings/lib/sync_handle_registry.cc
@@ -107,6 +107,11 @@ SyncHandleRegistry::SyncHandleRegistry() {
 
 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);
 }