Implement StereoPannerNode for robust stereo panning

Source/platform/audio/StereoPanner.h

Index: Source/platform/audio/StereoPanner.h
diff --git a/LICENSE b/Source/platform/audio/StereoPanner.h
similarity index 63%
copy from LICENSE
copy to Source/platform/audio/StereoPanner.h
index 70bcb8ad118978579fa055f7ecc99604930900ce..85a5f7b467294c150ddc8fcd0a5997c6d0d9d3e4 100644
--- a/LICENSE
+++ b/Source/platform/audio/StereoPanner.h
@@ -1,8 +1,5 @@
 // Copyright 2014 The Chromium Authors. All rights reserved.
 //
-// The Chromium Authors can be found at
-// http://src.chromium.org/svn/trunk/src/AUTHORS
-//
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
@@ -28,3 +25,35 @@
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#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:
+    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