Protect AudioScheduledSoureNode::m_startTime and m_endTime.

Source/modules/webaudio/AudioScheduledSourceNode.cpp

Index: Source/modules/webaudio/AudioScheduledSourceNode.cpp
diff --git a/Source/modules/webaudio/AudioScheduledSourceNode.cpp b/Source/modules/webaudio/AudioScheduledSourceNode.cpp
index d853761f1b983b2da6e9abf08075d2c58e9f99da..1e84f43b897081bb28045927340e5c3a28f2bd38 100644
--- a/Source/modules/webaudio/AudioScheduledSourceNode.cpp
+++ b/Source/modules/webaudio/AudioScheduledSourceNode.cpp
@@ -153,6 +153,10 @@ void AudioScheduledSourceHandler::start(double when, ExceptionState& exceptionSt
         return;
     }
 
+    // This synchronizes with process(). updateSchedulingInfo will read some of the variables being
+    // set here.
+    MutexLocker processLocker(m_processLock);
+
     // The node is started. Add a reference to keep us alive so that audio will eventually get
     // played even if Javascript should drop all references to this node. The reference will get
     // dropped when the source has finished playing.
@@ -186,6 +190,9 @@ void AudioScheduledSourceHandler::stop(double when, ExceptionState& exceptionSta
         return;
     }
 
+    // This synchronizes with process()
+    MutexLocker processLocker(m_processLock);
+
     // stop() can be called more than once, with the last call to stop taking effect, unless the
     // source has already stopped due to earlier calls to stop. No exceptions are thrown in any
     // case.