Treat a stopped AudioContext as closed.

Source/modules/webaudio/OfflineAudioContext.cpp

 /*
  * Copyright (C) 2012, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 83 matching lines @@
     : AudioContext(document, numberOfChannels, numberOfFrames, sampleRate)
 {
 }
 
 OfflineAudioContext::~OfflineAudioContext()
 {
 }
 
 ScriptPromise OfflineAudioContext::startOfflineRendering(ScriptState* scriptState)
 {
+    // Calling close() on an OfflineAudioContext is not supported/allowed,
+    // but it might well have been stopped by its execution context.
+    if (isContextClosed()) {
+        return ScriptPromise::rejectWithDOMException(
+            scriptState,
+            DOMException::create(
+                InvalidStateError,
+                "cannot call startRendering on an OfflineAudioContext in a stopped state."));
+    }
+
     if (m_offlineResolver) {
         // Can't call startRendering more than once.  Return a rejected promise now.
         return ScriptPromise::rejectWithDOMException(
             scriptState,
             DOMException::create(
                 InvalidStateError,
                 "cannot call startRendering more than once"));
     }
 
     m_offlineResolver = ScriptPromiseResolver::create(scriptState);
     startRendering();
     return m_offlineResolver->promise();
 }
 
 } // namespace blink
 
 #endif // ENABLE(WEB_AUDIO)