Use Document, rather than document element, for implicit root.

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 2389 matching lines @@
             'dom/DocumentType.cpp',
             'dom/DocumentVisibilityObserver.cpp',
             'dom/Element.cpp',
             'dom/Element.h',
             'dom/ElementData.cpp',
             'dom/ElementData.h',
             'dom/ElementDataCache.cpp',
             'dom/ElementDataCache.h',
             'dom/ElementFullscreen.cpp',
             'dom/ElementFullscreen.h',
-            'dom/ElementIntersectionObserverData.cpp',
-            'dom/ElementIntersectionObserverData.h',
             'dom/ElementRareData.cpp',
             'dom/ElementRareData.h',
             'dom/ElementTraversal.h',
             'dom/EmptyNodeList.cpp',
             'dom/EmptyNodeList.h',
             'dom/ExecutionContext.cpp',
             'dom/ExecutionContext.h',
             'dom/ExecutionContextTask.h',
             'dom/FirstLetterPseudoElement.cpp',
             'dom/FirstLetterPseudoElement.h',
@@ skipping 52 matching lines @@
             'dom/NameNodeList.cpp',
             'dom/NameNodeList.h',
             'dom/NamedNodeMap.cpp',
             'dom/Node.cpp',
             'dom/NodeChildRemovalTracker.cpp',
             'dom/NodeChildRemovalTracker.h',
             'dom/NodeComputedStyle.h',
             'dom/NodeFilter.cpp',
             'dom/NodeFilter.h',
             'dom/NodeFilterCondition.h',
+            'dom/NodeIntersectionObserverData.cpp',
+            'dom/NodeIntersectionObserverData.h',
             'dom/NodeIterator.cpp',
             'dom/NodeIterator.h',
             'dom/NodeIteratorBase.cpp',
             'dom/NodeIteratorBase.h',
             'dom/NodeList.cpp',
             'dom/NodeListsNodeData.cpp',
             'dom/NodeListsNodeData.h',
             'dom/NodeRareData.cpp',
             'dom/NodeRareData.h',
             'dom/NodeTraversal.cpp',
@@ skipping 1515 matching lines @@
             'style/SVGComputedStyleTest.cpp',
             'svg/SVGPathParserTest.cpp',
             'svg/UnsafeSVGAttributeSanitizationTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }