Add the CustomElementsRegistry interface behind the flag

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 97 matching lines @@
             'dom/Text.idl',
             'dom/Touch.idl',
             'dom/TouchList.idl',
             'dom/TreeWalker.idl',
             'dom/Uint16Array.idl',
             'dom/Uint32Array.idl',
             'dom/Uint8Array.idl',
             'dom/Uint8ClampedArray.idl',
             'dom/URLSearchParams.idl',
             'dom/XMLDocument.idl',
+            'dom/custom/CustomElementsRegistry.idl',
             'dom/shadow/ShadowRoot.idl',
             'editing/Selection.idl',
             'events/AnimationEvent.idl',
             'events/AnimationPlayerEvent.idl',
             'events/ApplicationCacheErrorEvent.idl',
             'events/AutocompleteErrorEvent.idl',
             'events/BeforeUnloadEvent.idl',
             'events/ClipboardEvent.idl',
             'events/CompositionEvent.idl',
             'events/CustomEvent.idl',
@@ skipping 2517 matching lines @@
             'dom/custom/CustomElementRegistrationContext.cpp',
             'dom/custom/CustomElementRegistrationContext.h',
             'dom/custom/CustomElementRegistry.cpp',
             'dom/custom/CustomElementRegistry.h',
             'dom/custom/CustomElementScheduler.cpp',
             'dom/custom/CustomElementScheduler.h',
             'dom/custom/CustomElementSyncMicrotaskQueue.cpp',
             'dom/custom/CustomElementSyncMicrotaskQueue.h',
             'dom/custom/CustomElementUpgradeCandidateMap.cpp',
             'dom/custom/CustomElementUpgradeCandidateMap.h',
+            'dom/custom/CustomElementsRegistry.cpp',
+            'dom/custom/CustomElementsRegistry.h',
             'dom/shadow/DistributedNodes.cpp',
             'dom/shadow/DistributedNodes.h',
             'dom/shadow/ElementShadow.cpp',
             'dom/shadow/ElementShadow.h',
             'dom/shadow/FlatTreeTraversal.cpp',
             'dom/shadow/FlatTreeTraversal.h',
             'dom/shadow/InsertionPoint.cpp',
             'dom/shadow/InsertionPoint.h',
             'dom/shadow/SelectRuleFeatureSet.cpp',
             'dom/shadow/SelectRuleFeatureSet.h',
@@ skipping 1421 matching lines @@
             'svg/SVGPathParserTest.cpp',
             'svg/UnsafeSVGAttributeSanitizationTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'workers/WorkerThreadTestHelper.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }