Add expansion of shorthands with custom properties to longhands using a pending substition value.

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 1179 matching lines @@
             'css/CSSNamespaceRule.cpp',
             'css/CSSNamespaceRule.h',
             'css/CSSPaintImageGenerator.cpp',
             'css/CSSPaintImageGenerator.h',
             'css/CSSPaintValue.cpp',
             'css/CSSPaintValue.h',
             'css/CSSPageRule.cpp',
             'css/CSSPageRule.h',
             'css/CSSPathValue.cpp',
             'css/CSSPathValue.h',
+            'css/CSSPendingSubstitutionValue.cpp',
+            'css/CSSPendingSubstitutionValue.h',
             'css/CSSPrimitiveValue.cpp',
             'css/CSSPrimitiveValue.h',
             'css/CSSPrimitiveValueMappings.h',
             'css/CSSProperty.cpp',
             'css/CSSPropertyEquality.cpp',
             'css/CSSPropertyEquality.h',
             'css/CSSPropertySourceData.cpp',
             'css/CSSQuadValue.cpp',
             'css/CSSQuadValue.h',
             'css/CSSReflectValue.cpp',
@@ skipping 2978 matching lines @@
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'timing/PerformanceBaseTest.cpp',
             'timing/PerformanceObserverTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'workers/WorkerThreadTestHelper.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }