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..752cf51e5c60596c875bd7b05d4f807f12e11e72 100644
--- a/Source/modules/webaudio/AudioScheduledSourceNode.h
+++ b/Source/modules/webaudio/AudioScheduledSourceNode.h
@@ -103,6 +103,33 @@ protected:
 
     void notifyEnded();
 
+    // Both |m_startTime| and |m_endTime| are accessed from both the main thread and the audio
+    // thread.  Use the setters and getters to protect the access to these!
+
+    double startTime() const
+    {
+        return acquireLoad(&m_startTime);

[tkent, 2015/08/07 01:10:03]

I'm not sure acquireLoad and releaseStore are enough for 64 bit value on 32 bit
platforms.

[Raymond Toy, 2015/08/07 15:45:00]

Hmm. I perhaps erroneously assumed floating-point reads/writes would be atomic
64-bit operations.  Do you have an alternative?  If this is not enough, I guess
a more coarse-grained approach will be needed. 

Atomics.h has support for long long, which should be 64 bits.  If that works,
floats and doubles should also work, right?

[tkent, 2015/08/11 00:45:19]

Alternative would be a normal mutex, or a spin lock.
It's possible that they block an audio rendering thread, but the block time must
be very short. 

+    }
+
+    void setStartTime(double newStartTime)
+    {
+        releaseStore(&m_startTime, newStartTime);
+    }
+
+    double endTime() const
+    {
+        return acquireLoad(&m_endTime);
+    }
+
+    void setEndTime(double newEndTime)
+    {
+        releaseStore(&m_endTime, newEndTime);
+    }
+
+    bool m_hasEndedListener;
+
+    static const double UnknownTime;
+private:
     // 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
 
@@ -111,10 +138,6 @@ protected:
     // has been reached.
     double m_endTime; // in seconds
 
-    bool m_hasEndedListener;
-
-    static const double UnknownTime;
-private:
     // This is accessed by both the main thread and audio thread.  Use the setter and getter to
     // protect the access to this!
     int m_playbackState;