Add MessageLoop::AssertIdle(). Use in ChromeOS audio threads.

base/message_loop.cc

Index: base/message_loop.cc
diff --git a/base/message_loop.cc b/base/message_loop.cc
index 518a0fd25989864f1dcbe539f10c4f51e5b44384..d83adccec08b56f8cdeb991c1b06af60d7a877e0 100644
--- a/base/message_loop.cc
+++ b/base/message_loop.cc
@@ -286,6 +286,12 @@ void MessageLoop::RemoveTaskObserver(TaskObserver* task_observer) {
   task_observers_.RemoveObserver(task_observer);
 }
 
+void MessageLoop::AssertIdle() const {
+  // We only check |incoming_queue_|, since we don't want to lock |work_queue_|.
+  base::AutoLock lock(incoming_queue_lock_);
+  DCHECK(incoming_queue_.empty());
+}
+
 //------------------------------------------------------------------------------
 
 // Runs the loop in two different SEH modes: