Fix some MultiplexRouter behavior for sync messages

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

Index: mojo/public/cpp/bindings/lib/multiplex_router.cc
diff --git a/mojo/public/cpp/bindings/lib/multiplex_router.cc b/mojo/public/cpp/bindings/lib/multiplex_router.cc
index d081b0155ba480edf20fd4357b00f4d49f7ecb62..8b461ea2e92eff43fa2c8c162c5ae0dd9b45891f 100644
--- a/mojo/public/cpp/bindings/lib/multiplex_router.cc
+++ b/mojo/public/cpp/bindings/lib/multiplex_router.cc
@@ -594,7 +594,7 @@ void MultiplexRouter::ProcessTasks(
     bool sync_message = task->IsMessageTask() && task->message &&
                         task->message->has_flag(kMessageIsSync);
     if (sync_message) {
-      InterfaceId id = task->message->interface_id();
+      id = task->message->interface_id();
       auto& sync_message_queue = sync_message_tasks_[id];
       DCHECK_EQ(task.get(), sync_message_queue.front());
       sync_message_queue.pop_front();
@@ -608,11 +608,11 @@ void MultiplexRouter::ProcessTasks(
                                      current_task_runner);
 
     if (!processed) {
-      tasks_.push_front(std::move(task));
       if (sync_message) {
         auto& sync_message_queue = sync_message_tasks_[id];
         sync_message_queue.push_front(task.get());
       }
+      tasks_.push_front(std::move(task));
       break;
     } else {
       if (sync_message) {
@@ -643,7 +643,15 @@ bool MultiplexRouter::ProcessFirstSyncMessageForEndpoint(InterfaceId id) {
   DCHECK(processed);
 
   iter = sync_message_tasks_.find(id);
-  return iter != sync_message_tasks_.end() && !iter->second.empty();
+  if (iter == sync_message_tasks_.end())
+    return false;
+
+  if (iter->second.empty()) {
+    sync_message_tasks_.erase(iter);
+    return false;
+  }
+
+  return true;
 }
 
 bool MultiplexRouter::ProcessNotifyErrorTask(