Protect AudioScheduledSoureNode::m_startTime and m_endTime.

Source/modules/webaudio/AudioScheduledSourceNode.h

Index: Source/modules/webaudio/AudioScheduledSourceNode.h
diff --git a/Source/modules/webaudio/AudioScheduledSourceNode.h b/Source/modules/webaudio/AudioScheduledSourceNode.h
index 11c3e5196ff65c116e8ad6e1ebc1b4e48cf37065..9e36c1bea247142efff9e7b1b4e17db306b3534b 100644
--- a/Source/modules/webaudio/AudioScheduledSourceNode.h
+++ b/Source/modules/webaudio/AudioScheduledSourceNode.h
@@ -103,6 +103,10 @@ protected:
 
     void notifyEnded();
 
+    // This synchronizes with process() and any other method that needs to be synchronized like
+    // setBuffer for AudioBufferSource.
+    mutable Mutex m_processLock;
+
     // m_startTime is the time to start playing based on the context's timeline (0 or a time less than the context's current time means "now").
     double m_startTime; // in seconds