Add thread safety to AudioManagerBase to protect access to the audio thread member variable.

content/test/webrtc_audio_device_test.cc

Index: content/test/webrtc_audio_device_test.cc
diff --git a/content/test/webrtc_audio_device_test.cc b/content/test/webrtc_audio_device_test.cc
index d4ea8b7d71f15fa33162bb27bd7368f0872ecbd2..68cb8eaecd830fc2cb10354040e8ca0b047ec27e 100644
--- a/content/test/webrtc_audio_device_test.cc
+++ b/content/test/webrtc_audio_device_test.cc
@@ -277,7 +277,8 @@ bool WebRTCAudioDeviceTest::OnMessageReceived(const IPC::Message& message) {
 
 // Posts a final task to the IO message loop and waits for completion.
 void WebRTCAudioDeviceTest::WaitForIOThreadCompletion() {
-  WaitForMessageLoopCompletion(ChildProcess::current()->io_message_loop());
+  WaitForMessageLoopCompletion(
+      ChildProcess::current()->io_message_loop()->message_loop_proxy());
 }
 
 void WebRTCAudioDeviceTest::WaitForAudioManagerCompletion() {
@@ -285,7 +286,8 @@ void WebRTCAudioDeviceTest::WaitForAudioManagerCompletion() {
     WaitForMessageLoopCompletion(audio_manager_->GetMessageLoop());
 }
 
-void WebRTCAudioDeviceTest::WaitForMessageLoopCompletion(MessageLoop* loop) {
+void WebRTCAudioDeviceTest::WaitForMessageLoopCompletion(
+    base::MessageLoopProxy* loop) {
   base::WaitableEvent* event = new base::WaitableEvent(false, false);
   loop->PostTask(FROM_HERE, base::Bind(&base::WaitableEvent::Signal,
                  base::Unretained(event)));