Clear DeferredTaskHandler's audio thread ID upon the thread going away.

third_party/WebKit/Source/modules/webaudio/DeferredTaskHandler.cpp

Index: third_party/WebKit/Source/modules/webaudio/DeferredTaskHandler.cpp
diff --git a/third_party/WebKit/Source/modules/webaudio/DeferredTaskHandler.cpp b/third_party/WebKit/Source/modules/webaudio/DeferredTaskHandler.cpp
index 08a748884849b175b2db7d0d74e583f10c3396fd..d294fe8fa80585e76e0331a63cb657d395b0f72c 100644
--- a/third_party/WebKit/Source/modules/webaudio/DeferredTaskHandler.cpp
+++ b/third_party/WebKit/Source/modules/webaudio/DeferredTaskHandler.cpp
@@ -279,6 +279,11 @@ void DeferredTaskHandler::clearHandlersToBeDeleted()
     m_deletableOrphanHandlers.clear();
 }
 
+void DeferredTaskHandler::clearAudioThread()
+{
+    releaseStore(&m_audioThread, 0);
+}
+
 void DeferredTaskHandler::setAudioThreadToCurrentThread()
 {
     ASSERT(!isMainThread());