Monitor the IO message loop in the AudioDevice classes.

content/renderer/media/scoped_loop_observer.cc

+// Copyright (c) 2012 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 "content/renderer/media/scoped_loop_observer.h"
+
+#include "base/bind.h"
+#include "base/synchronization/waitable_event.h"
+
+ScopedLoopObserver::ScopedLoopObserver(MessageLoop* loop)
+    : loop_(loop->message_loop_proxy()) {
+  ObserveLoopDestruction(true, NULL);
+}
+
+ScopedLoopObserver::~ScopedLoopObserver() {
+  ObserveLoopDestruction(false, NULL);
+}
+
+void ScopedLoopObserver::ObserveLoopDestruction(bool enable,
+                                                base::WaitableEvent* done) {
+  // Note: |done| may be NULL.
+  if (loop_->BelongsToCurrentThread()) {
+    MessageLoop* loop = MessageLoop::current();
+    if (enable) {
+      loop->AddDestructionObserver(this);
+    } else {
+      loop->RemoveDestructionObserver(this);
+    }
+  } else {
+    base::WaitableEvent event(false, false);
+    if (loop_->PostTask(FROM_HERE,
+            base::Bind(&ScopedLoopObserver::ObserveLoopDestruction,
+                       base::Unretained(this), enable, &event))) {
+      event.Wait();
+    } else {
+      // The message loop's thread has already terminated, so no need to wait.
+    }
+  }
+
+  if (done)
+    done->Signal();
+}