Disallow cross-thread Persistent<> read access.

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

Index: third_party/WebKit/Source/modules/webaudio/AudioBufferSourceNode.h
diff --git a/third_party/WebKit/Source/modules/webaudio/AudioBufferSourceNode.h b/third_party/WebKit/Source/modules/webaudio/AudioBufferSourceNode.h
index 2d2739acb3fb2b9a6e91ee3f5f90a2ae8090737b..3e506c199a59713db922e64f0fafd7ad721b84a4 100644
--- a/third_party/WebKit/Source/modules/webaudio/AudioBufferSourceNode.h
+++ b/third_party/WebKit/Source/modules/webaudio/AudioBufferSourceNode.h
@@ -124,7 +124,8 @@ class AudioBufferSourceHandler final : public AudioScheduledSourceHandler {
   // m_buffer holds the sample data which this node outputs.
   // This Persistent doesn't make a reference cycle including
   // AudioBufferSourceNode.
-  Persistent<AudioBuffer> m_buffer;
+  // It is cross-thread, as it will be accessed by the audio and main threads.
+  CrossThreadPersistent<AudioBuffer> m_buffer;
 
   // Pointers for the buffer and destination.
   std::unique_ptr<const float* []> m_sourceChannels;