Implement StereoPannerNode for robust stereo panning

Source/modules/modules.gypi

 {
   'includes': [
     '../core/core_generated.gypi',
     'modules_generated.gypi',
   ],
   'variables': {
     # Experimental hooks for embedder to provide extra IDL and source files.
     #
     # Note: this is not a supported API. If you rely on this, you will be broken
     # from time to time as the code generator changes in backward incompatible
@@ skipping 166 matching lines @@
       'webaudio/GainNode.idl',
       'webaudio/MediaElementAudioSourceNode.idl',
       'webaudio/MediaStreamAudioDestinationNode.idl',
       'webaudio/MediaStreamAudioSourceNode.idl',
       'webaudio/OfflineAudioCompletionEvent.idl',
       'webaudio/OfflineAudioContext.idl',
       'webaudio/OscillatorNode.idl',
       'webaudio/PannerNode.idl',
       'webaudio/PeriodicWave.idl',
       'webaudio/ScriptProcessorNode.idl',
+      'webaudio/StereoPannerNode.idl',
       'webaudio/WaveShaperNode.idl',
       'webdatabase/Database.idl',
       'webdatabase/DatabaseCallback.idl',
       'webdatabase/SQLError.idl',
       'webdatabase/SQLResultSet.idl',
       'webdatabase/SQLResultSetRowList.idl',
       'webdatabase/SQLStatementCallback.idl',
       'webdatabase/SQLStatementErrorCallback.idl',
       'webdatabase/SQLTransaction.idl',
       'webdatabase/SQLTransactionCallback.idl',
@@ skipping 771 matching lines @@
       'webaudio/OscillatorNode.cpp',
       'webaudio/OscillatorNode.h',
       'webaudio/PannerNode.cpp',
       'webaudio/PannerNode.h',
       'webaudio/PeriodicWave.cpp',
       'webaudio/PeriodicWave.h',
       'webaudio/RealtimeAnalyser.cpp',
       'webaudio/RealtimeAnalyser.h',
       'webaudio/ScriptProcessorNode.cpp',
       'webaudio/ScriptProcessorNode.h',
+      'webaudio/StereoPannerNode.cpp',
+      'webaudio/StereoPannerNode.h',
       'webaudio/WaveShaperDSPKernel.cpp',
       'webaudio/WaveShaperDSPKernel.h',
       'webaudio/WaveShaperNode.cpp',
       'webaudio/WaveShaperNode.h',
       'webaudio/WaveShaperProcessor.cpp',
       'webaudio/WaveShaperProcessor.h',
       'webdatabase/ChangeVersionData.h',
       'webdatabase/ChangeVersionWrapper.cpp',
       'webdatabase/ChangeVersionWrapper.h',
       'webdatabase/DOMWindowWebDatabase.cpp',
@@ skipping 128 matching lines @@
       'serviceworkers/CacheTest.cpp',
       'serviceworkers/FetchResponseDataTest.cpp',
       'serviceworkers/RequestTest.cpp',
       'serviceworkers/ResponseTest.cpp',
       'serviceworkers/ServiceWorkerContainerTest.cpp',
       'websockets/DOMWebSocketTest.cpp',
       'websockets/DocumentWebSocketChannelTest.cpp',
     ],
   },
 }