Implement StereoPannerNode for robust stereo panning

Source/platform/audio/StereoPanner.h

Index: Source/platform/audio/StereoPanner.h
diff --git a/Source/platform/audio/StereoPanner.h b/Source/platform/audio/StereoPanner.h
new file mode 100644
index 0000000000000000000000000000000000000000..9d9b7f826a785d018d6dc6c8da8f394998d0752a
--- /dev/null
+++ b/Source/platform/audio/StereoPanner.h
@@ -0,0 +1,35 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef StereoPanner_h
+#define StereoPanner_h
+
+#include "platform/audio/Spatializer.h"
+
+namespace blink {
+
+// Common type of stereo panner as found in normal audio mixing equipment.
+class PLATFORM_EXPORT StereoPanner final : public Spatializer {
+public:
+    explicit StereoPanner(float sampleRate);
+
+    virtual void panWithSampleAccurateValues(const AudioBus* inputBus, AudioBus* outputBuf, const float* panValues, size_t framesToProcess) override;
+    virtual void panToTargetValue(const AudioBus* inputBus, AudioBus* outputBuf, float panValue, size_t framesToProcess) override;
+
+    virtual void reset() override { }
+
+    virtual double tailTime() const override { return 0; }
+    virtual double latencyTime() const override { return 0; }
+
+private:
+    bool m_isFirstRender;
+    double m_smoothingConstant;
+
+    double m_gainL;
+    double m_gainR;
+};
+
+} // namespace blink
+
+#endif // StereoPanner_h