Implement StereoPannerNode for robust stereo panning

Source/modules/webaudio/StereoPannerNode.h

Index: Source/modules/webaudio/StereoPannerNode.h
diff --git a/Source/modules/webaudio/StereoPannerNode.h b/Source/modules/webaudio/StereoPannerNode.h
new file mode 100644
index 0000000000000000000000000000000000000000..e0bfec9b977acdcaee8058d866466ecfc204b9a9
--- /dev/null
+++ b/Source/modules/webaudio/StereoPannerNode.h
@@ -0,0 +1,53 @@
+// 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 StereoPannerNode_h
+#define StereoPannerNode_h
+
+#include "modules/webaudio/AudioNode.h"
+#include "modules/webaudio/AudioParam.h"
+#include "platform/audio/AudioBus.h"
+#include "platform/audio/Spatializer.h"
+
+namespace blink {
+
+// StereoPannerNode is an AudioNode with one input and one output. It is
+// specifically designed for equal-power stereo panning.
+class StereoPannerNode final : public AudioNode {
+    DEFINE_WRAPPERTYPEINFO();
+public:
+    static StereoPannerNode* create(AudioContext* context, float sampleRate)
+    {
+        return new StereoPannerNode(context, sampleRate);
+    }
+
+    virtual ~StereoPannerNode();
+
+    virtual void dispose() override;
+    virtual void process(size_t framesToProcess) override;
+    virtual void initialize() override;
+    virtual void uninitialize() override;
+
+    virtual double tailTime() const override { return 0; }
+    virtual double latencyTime() const override { return 0; }
+
+    virtual void setChannelCount(unsigned long, ExceptionState&) final;
+    virtual void setChannelCountMode(const String&, ExceptionState&) final;
+
+    virtual void trace(Visitor*) override;
+
+    AudioParam* pan() { return m_pan.get(); }
+
+private:
+    StereoPannerNode(AudioContext*, float sampleRate);
+
+    Member<Spatializer> m_stereoPanner;
+    Member<AudioParam> m_pan;
+
+    AudioFloatArray m_sampleAccuratePanValues;
+};
+
+} // namespace blink
+
+#endif // StereoPannerNode_h