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..39c30655e0e75e4b3b1eb898ce65a9e3d6759637 100644
--- a/Source/modules/webaudio/AudioBufferSourceNode.cpp
+++ b/Source/modules/webaudio/AudioBufferSourceNode.cpp
@@ -414,7 +414,7 @@ void AudioBufferSourceHandler::clampGrainParameters(const AudioBuffer* buffer)
         // grainDuration is larger than the buffer duration. The net effect is as if the user called
         // stop(when + grainDuration).
         m_grainDuration = clampTo(m_grainDuration, 0.0, std::numeric_limits<double>::infinity());
-        m_endTime = m_startTime + m_grainDuration;
+        setEndTime(startTime() + m_grainDuration);
     } else {
         m_grainDuration = clampTo(m_grainDuration, 0.0, bufferDuration - m_grainOffset);
     }
@@ -495,7 +495,7 @@ void AudioBufferSourceHandler::startSource(double when, double grainOffset, doub
 
     // If |when| < currentTime, the source must start now according to the spec.
     // So just set startTime to currentTime in this case to start the source now.
-    m_startTime = std::max(when, context()->currentTime());
+    setStartTime(std::max(when, context()->currentTime()));
 
     if (buffer())
         clampGrainParameters(buffer());
@@ -581,7 +581,7 @@ void AudioBufferSourceHandler::handleStoppableSourceNode()
         // ok.
         double actualDuration = buffer()->duration() / m_minPlaybackRate;
 
-        double stopTime = m_startTime + actualDuration;
+        double stopTime = startTime() + actualDuration;
 
         // See crbug.com/478301. If a source node is started via start(), the source may not start
         // at that time but one quantum (128 frames) later.  But we compute the stop time based on