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

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

 // Copyright 2015 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 "modules/webaudio/AudioContext.h"
 
 #include "bindings/core/v8/ExceptionMessages.h"
 #include "bindings/core/v8/ExceptionState.h"
 #include "bindings/core/v8/ScriptPromiseResolver.h"
 #include "core/dom/DOMException.h"
@@ skipping 176 matching lines @@
 
 void AudioContext::stopRendering()
 {
     ASSERT(isMainThread());
     ASSERT(destination());
 
     if (contextState() == Running) {
         destination()->audioDestinationHandler().stopRendering();
         setContextState(Suspended);
         deferredTaskHandler().clearHandlersToBeDeleted();
+        // The recorded audio thread will be shut down as part
+        // of stopping, so clear that association to avoid
+        // future confusion (crbug.com/612127)
+        deferredTaskHandler().clearAudioThread();
     }
 }
 
 } // namespace blink