Split out String, URI and CustomIdent from CSSPrimitiveValue

Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/KeyframeEffect.idl',
             'animation/EffectModel.idl',
             'animation/AnimationEffectReadOnly.idl',
@@ skipping 1080 matching lines @@
             'css/CSSRuleList.h',
             'css/CSSSVGDocumentValue.cpp',
             'css/CSSSVGDocumentValue.h',
             'css/CSSSegmentedFontFace.cpp',
             'css/CSSSegmentedFontFace.h',
             'css/CSSSelector.cpp',
             'css/CSSSelectorList.cpp',
             'css/CSSSelectorList.h',
             'css/CSSShadowValue.cpp',
             'css/CSSShadowValue.h',
+            'css/CSSStringValueBase.h',
+            'css/CSSStringValueBase.cpp',
             'css/CSSStyleDeclaration.h',
             'css/CSSStyleRule.cpp',
             'css/CSSStyleRule.h',
             'css/CSSStyleSheet.cpp',
             'css/CSSStyleSheet.h',
             'css/CSSSupportsRule.cpp',
             'css/CSSSupportsRule.h',
             'css/CSSTimingFunctionValue.cpp',
             'css/CSSTimingFunctionValue.h',
             'css/CSSToLengthConversionData.cpp',
@@ skipping 2760 matching lines @@
             'style/OutlineValueTest.cpp',
             'style/SVGComputedStyleTest.cpp',
             'svg/UnsafeSVGAttributeSanitizationTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }