Disallow cross-thread Persistent<> read access.

third_party/WebKit/Source/modules/webaudio/OscillatorNode.h

Index: third_party/WebKit/Source/modules/webaudio/OscillatorNode.h
diff --git a/third_party/WebKit/Source/modules/webaudio/OscillatorNode.h b/third_party/WebKit/Source/modules/webaudio/OscillatorNode.h
index bc8df7236c01012999437172d2114fcdd1b1d46e..207e2dd565250a6a289343628830e162738f4f18 100644
--- a/third_party/WebKit/Source/modules/webaudio/OscillatorNode.h
+++ b/third_party/WebKit/Source/modules/webaudio/OscillatorNode.h
@@ -96,8 +96,9 @@ class OscillatorHandler final : public AudioScheduledSourceHandler {
   AudioFloatArray m_detuneValues;
 
   // This Persistent doesn't make a reference cycle including the owner
-  // OscillatorNode.
-  Persistent<PeriodicWave> m_periodicWave;
+  // OscillatorNode. It is cross-thread, as it will be accessed by the audio
+  // thread.
+  CrossThreadPersistent<PeriodicWave> m_periodicWave;
 };
 
 class OscillatorNode final : public AudioScheduledSourceNode {