Introduce a SVGUnknownElement class for unknown SVG elements

Source/core/core.gypi

 {
     'variables': {
         'deprecated_perl_core_idl_files': [
             'css/CSS.idl',
             'css/CSSCharsetRule.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSHostRule.idl',
             'css/CSSImportRule.idl',
             'css/CSSMediaRule.idl',
@@ skipping 3594 matching lines @@
             'svg/SVGTitleElement.h',
             'svg/SVGTransform.cpp',
             'svg/SVGTransformDistance.cpp',
             'svg/SVGTransformDistance.h',
             'svg/SVGTransformList.cpp',
             'svg/SVGTransformable.cpp',
             'svg/SVGTransformable.h',
             'svg/SVGURIReference.cpp',
             'svg/SVGURIReference.h',
             'svg/SVGUnitTypes.h',
+            'svg/SVGUnknownElement.cpp',
+            'svg/SVGUnknownElement.h',
             'svg/SVGUseElement.cpp',
             'svg/SVGUseElement.h',
             'svg/SVGVKernElement.cpp',
             'svg/SVGVKernElement.h',
             'svg/SVGViewElement.cpp',
             'svg/SVGViewElement.h',
             'svg/SVGViewSpec.cpp',
             'svg/SVGViewSpec.h',
             'svg/SVGZoomAndPan.cpp',
             'svg/SVGZoomAndPan.h',
@@ skipping 176 matching lines @@
                 'bison_exe': 'bison',
                 # We specify a preprocess so it happens locally and won't get
                 # distributed to goma.
                 # FIXME: /usr/bin/gcc won't exist on OSX forever. We want to
                 # use /usr/bin/clang once we require Xcode 4.x.
                 'preprocessor': '--preprocessor "/usr/bin/gcc -E -P -x c++"'
               }],
          ],
     }
 }