Disallow cross-thread Persistent<> read access.

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

Index: third_party/WebKit/Source/modules/webaudio/ConvolverNode.h
diff --git a/third_party/WebKit/Source/modules/webaudio/ConvolverNode.h b/third_party/WebKit/Source/modules/webaudio/ConvolverNode.h
index b87197ece9675be86781a1b5b78d01dc59b51dff..90857b3f8c424653680cdba1ad4d123cdcfb51ac 100644
--- a/third_party/WebKit/Source/modules/webaudio/ConvolverNode.h
+++ b/third_party/WebKit/Source/modules/webaudio/ConvolverNode.h
@@ -63,7 +63,8 @@ class MODULES_EXPORT ConvolverHandler final : public AudioHandler {
   std::unique_ptr<Reverb> m_reverb;
   // This Persistent doesn't make a reference cycle including the owner
   // ConvolverNode.
-  Persistent<AudioBuffer> m_buffer;
+  // It is cross-thread, as it will be accessed by the audio and main threads.
+  CrossThreadPersistent<AudioBuffer> m_buffer;
 
   // This synchronizes dynamic changes to the convolution impulse response with
   // process().