Add paint() function as valid CSS <image> type.

third_party/WebKit/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 1146 matching lines @@
             'css/CSSKeyframesRule.cpp',
             'css/CSSKeyframesRule.h',
             'css/CSSMarkup.cpp',
             'css/CSSMarkup.h',
             'css/CSSMatrix.cpp',
             'css/CSSMatrix.h',
             'css/CSSMediaRule.cpp',
             'css/CSSMediaRule.h',
             'css/CSSNamespaceRule.cpp',
             'css/CSSNamespaceRule.h',
+            'css/CSSPaintValue.cpp',
+            'css/CSSPaintValue.h',
             'css/CSSPageRule.cpp',
             'css/CSSPageRule.h',
             'css/CSSPathValue.cpp',
             'css/CSSPathValue.h',
             'css/CSSPrimitiveValue.cpp',
             'css/CSSPrimitiveValueMappings.h',
             'css/CSSProperty.cpp',
             'css/CSSPropertySourceData.cpp',
             'css/CSSQuadValue.cpp',
             'css/CSSQuadValue.h',
@@ skipping 2887 matching lines @@
             'style/SVGComputedStyleTest.cpp',
             'svg/SVGPathParserTest.cpp',
             'svg/UnsafeSVGAttributeSanitizationTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }