Implement StereoPannerNode for robust stereo panning

Source/platform/audio/StereoPanner.h

+// 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