Implement custom element construction and some 'define' checks

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 2612 matching lines @@
             'dom/UserActionElementSet.h',
             'dom/ViewportDescription.cpp',
             'dom/ViewportDescription.h',
             'dom/VisitedLinkState.cpp',
             'dom/VisitedLinkState.h',
             'dom/WeakIdentifierMap.h',
             'dom/XMLDocument.cpp',
             'dom/XMLDocument.h',
             'dom/custom/CustomElement.cpp',
             'dom/custom/CustomElement.h',
+            'dom/custom/CustomElementDefinition.cpp',
+            'dom/custom/CustomElementDefinition.h',
             'dom/custom/CustomElementsRegistry.cpp',
             'dom/custom/CustomElementsRegistry.h',
             'dom/custom/V0CustomElement.cpp',
             'dom/custom/V0CustomElement.h',
             'dom/custom/V0CustomElementAsyncImportMicrotaskQueue.cpp',
             'dom/custom/V0CustomElementAsyncImportMicrotaskQueue.h',
             'dom/custom/V0CustomElementCallbackInvocation.cpp',
             'dom/custom/V0CustomElementCallbackInvocation.h',
             'dom/custom/V0CustomElementCallbackQueue.cpp',
             'dom/custom/V0CustomElementCallbackQueue.h',
@@ skipping 1480 matching lines @@
             'svg/UnsafeSVGAttributeSanitizationTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'timing/PerformanceBaseTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'workers/WorkerThreadTestHelper.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }