Access current frame counter carefully and remove m_cachedSampleFrame.

Source/modules/webaudio/AudioDestinationNode.cpp

Index: Source/modules/webaudio/AudioDestinationNode.cpp
diff --git a/Source/modules/webaudio/AudioDestinationNode.cpp b/Source/modules/webaudio/AudioDestinationNode.cpp
index 538a366a9c16e6424baac2ee29d0c6c9190e4e4e..b061e8a81e5f35055a1c7a9d6a8ffd24094fd8ce 100644
--- a/Source/modules/webaudio/AudioDestinationNode.cpp
+++ b/Source/modules/webaudio/AudioDestinationNode.cpp
@@ -31,6 +31,7 @@
 #include "modules/webaudio/AudioNodeOutput.h"
 #include "platform/audio/AudioUtilities.h"
 #include "platform/audio/DenormalDisabler.h"
+#include "wtf/Atomics.h"
 
 namespace blink {
 
@@ -90,7 +91,7 @@ void AudioDestinationHandler::render(AudioBus* sourceBus, AudioBus* destinationB
     context()->handlePostRenderTasks();
 
     // Advance current sample-frame.
-    m_currentSampleFrame += numberOfFrames;
+    atomicAdd(&m_currentSampleFrame, numberOfFrames);

[tkent, 2015/05/22 00:47:12]

I don't think using atomicAdd() here is correct.
atomicAdd() is implemented by InterlockedExchangeAdd() on Windows, and 
InterlockedExchangeAdd() avoid races with other Interlocked* functions.  So,
accessing m_currentSampleFrame directly from the main thread is not safe.
We have only one audio thread for a single AudioDestinationHandler.  We don't
need to use atomicAdd() here.

Instead, we should do:

AudioDestinationHandler::render():
    size_t sampleFrame = m_currentSampleFrame + numberOfFrames;
    releaseStore(&m_currentSampleFrame, sampleFrame);

AudioDestinationHandler::rcurrentSampleFrame():
    return acquireLoad(&m_currentSampleFrame);

[Raymond Toy, 2015/05/22 16:13:48]

Done.  Thanks for the hint.

 }
 
 // ----------------------------------------------------------------