Remove support for the externalResourcesRequired attribute.

Source/core/core.gypi

 {
     'variables': {
         'core_idl_files': [
             'css/CSS.idl',
             'css/CSSCharsetRule.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSHostRule.idl',
             'css/CSSImportRule.idl',
             'css/CSSKeyframeRule.idl',
@@ skipping 339 matching lines @@
             'svg/SVGColor.idl',
             'svg/SVGComponentTransferFunctionElement.idl',
             'svg/SVGCursorElement.idl',
             'svg/SVGDefsElement.idl',
             'svg/SVGDescElement.idl',
             'svg/SVGDocument.idl',
             'svg/SVGElement.idl',
             'svg/SVGElementInstance.idl',
             'svg/SVGElementInstanceList.idl',
             'svg/SVGEllipseElement.idl',
-            'svg/SVGExternalResourcesRequired.idl',
             'svg/SVGFEBlendElement.idl',
             'svg/SVGFEColorMatrixElement.idl',
             'svg/SVGFEComponentTransferElement.idl',
             'svg/SVGFECompositeElement.idl',
             'svg/SVGFEConvolveMatrixElement.idl',
             'svg/SVGFEDiffuseLightingElement.idl',
             'svg/SVGFEDisplacementMapElement.idl',
             'svg/SVGFEDistantLightElement.idl',
             'svg/SVGFEDropShadowElement.idl',
             'svg/SVGFEFloodElement.idl',
@@ skipping 2839 matching lines @@
             'svg/SVGDocument.h',
             'svg/SVGDocumentExtensions.cpp',
             'svg/SVGElement.cpp',
             'svg/SVGElementInstance.cpp',
             'svg/SVGElementInstance.h',
             'svg/SVGElementInstanceList.cpp',
             'svg/SVGElementInstanceList.h',
             'svg/SVGElementRareData.h',
             'svg/SVGEllipseElement.cpp',
             'svg/SVGEllipseElement.h',
-            'svg/SVGExternalResourcesRequired.cpp',
-            'svg/SVGExternalResourcesRequired.h',
             'svg/SVGFEBlendElement.cpp',
             'svg/SVGFEBlendElement.h',
             'svg/SVGFEColorMatrixElement.cpp',
             'svg/SVGFEColorMatrixElement.h',
             'svg/SVGFEComponentTransferElement.cpp',
             'svg/SVGFEComponentTransferElement.h',
             'svg/SVGFECompositeElement.cpp',
             'svg/SVGFECompositeElement.h',
             'svg/SVGFEConvolveMatrixElement.cpp',
             'svg/SVGFEConvolveMatrixElement.h',
@@ skipping 393 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++"'
               }],
          ],
     }
 }