Implement StereoPannerNode for robust stereo panning

Source/modules/webaudio/StereoPannerNode.idl

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-// The Chromium Authors can be found at
-// http://src.chromium.org/svn/trunk/src/AUTHORS
-//
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[Raymond Toy, 2014/11/12 18:06:52]

The coding style indicates that the license header is just 3 lines long, and the
rest of this is not needed.

Be sure to do the same thing for all of the new files.

[hongchan, 2014/11/12 22:01:43]

Done.

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 // met:
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+
+[
+    NoInterfaceObject,
+    Conditional=WEB_AUDIO
+] interface StereoPannerNode : AudioNode {
+
+    readonly attribute AudioParam pan;
+
+};