Protect AudioScheduledSoureNode::m_startTime and m_endTime.

Source/modules/webaudio/AudioBufferSourceNode.cpp

Index: Source/modules/webaudio/AudioBufferSourceNode.cpp
diff --git a/Source/modules/webaudio/AudioBufferSourceNode.cpp b/Source/modules/webaudio/AudioBufferSourceNode.cpp
index 98824d94f3470554897b98e1bc44f6c61ec698ba..c2e8e9bbb9ec428c2f6b0c9ead26d254f44a353e 100644
--- a/Source/modules/webaudio/AudioBufferSourceNode.cpp
+++ b/Source/modules/webaudio/AudioBufferSourceNode.cpp
@@ -482,6 +482,10 @@ void AudioBufferSourceHandler::startSource(double when, double grainOffset, doub
         return;
     }
 
+    // This synchronizes with process(). updateSchedulingInfo will read some of the variables being
+    // set here.
+    MutexLocker processLocker(m_processLock);
+
     m_isDurationGiven = isDurationGiven;
     m_isGrain = true;
     m_grainOffset = grainOffset;