Implement StereoPannerNode for robust stereo panning

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 214 matching lines @@
       'audio/ReverbAccumulationBuffer.cpp',
       'audio/ReverbAccumulationBuffer.h',
       'audio/ReverbConvolver.cpp',
       'audio/ReverbConvolver.h',
       'audio/ReverbConvolverStage.cpp',
       'audio/ReverbConvolverStage.h',
       'audio/ReverbInputBuffer.cpp',
       'audio/ReverbInputBuffer.h',
       'audio/SincResampler.cpp',
       'audio/SincResampler.h',
+      'audio/StereoPanner.cpp',
+      'audio/StereoPanner.h',
+      'audio/Spatializer.cpp',
+      'audio/Spatializer.h',
       'audio/UpSampler.cpp',
       'audio/UpSampler.h',
       'audio/VectorMath.cpp',
       'audio/VectorMath.h',
       'audio/ZeroPole.cpp',
       'audio/ZeroPole.h',
       'audio/android/FFTFrameOpenMAXDLAndroid.cpp',
       'audio/ffmpeg/FFTFrameFFMPEG.cpp',
       'audio/ipp/FFTFrameIPP.cpp',
       'audio/mac/FFTFrameMac.cpp',
@@ skipping 698 matching lines @@
       'graphics/ImageFrameGeneratorTest.cpp',
       'graphics/ImageLayerChromiumTest.cpp',
       'graphics/test/MockImageDecoder.h',
       'graphics/test/MockWebGraphicsContext3D.h',
       'image-decoders/gif/GIFImageDecoderTest.cpp',
       'image-decoders/jpeg/JPEGImageDecoderTest.cpp',
       'image-decoders/webp/WEBPImageDecoderTest.cpp',
     ],
   },
 }