Protect AudioScheduledSoureNode::m_startTime and m_endTime.

Source/modules/webaudio/AudioBufferSourceNode.h

Index: Source/modules/webaudio/AudioBufferSourceNode.h
diff --git a/Source/modules/webaudio/AudioBufferSourceNode.h b/Source/modules/webaudio/AudioBufferSourceNode.h
index 38266d8c3877cbfe9fd15c2c9b3480c7507277e1..bcfc07fd85ceae250ea0eecbbe3553c59d861b9e 100644
--- a/Source/modules/webaudio/AudioBufferSourceNode.h
+++ b/Source/modules/webaudio/AudioBufferSourceNode.h
@@ -145,9 +145,6 @@ private:
     // AudioBufferSourceNode should have Member<PannerNode>.
     RefPtr<PannerHandler> m_pannerNode;
 
-    // This synchronizes process() with setBuffer() which can cause dynamic channel count changes.
-    mutable Mutex m_processLock;
-
     // The minimum playbackRate value ever used for this source.  This includes any adjustments
     // caused by doppler too.
     double m_minPlaybackRate;