Move LiveNodeListBase class to its own header

Source/core/core.gypi

 {
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/Player.idl',
             'animation/TimedItem.idl',
             'animation/Timeline.idl',
             'animation/Timing.idl',
             'clipboard/Clipboard.idl',
@@ skipping 1915 matching lines @@
             'dom/SiblingRuleHelper.h',
             'dom/IconURL.cpp',
             'dom/IconURL.h',
             'dom/IdTargetObserver.cpp',
             'dom/IdTargetObserver.h',
             'dom/IdTargetObserverRegistry.cpp',
             'dom/IdTargetObserverRegistry.h',
             'dom/IgnoreDestructiveWriteCountIncrementer.h',
             'dom/LiveNodeList.cpp',
             'dom/LiveNodeList.h',
+            'dom/LiveNodeListBase.cpp',
+            'dom/LiveNodeListBase.h',
             'dom/MessageChannel.cpp',
             'dom/MessageChannel.h',
             'dom/MessagePort.cpp',
             'dom/Microtask.cpp',
             'dom/Microtask.h',
             'dom/MutationCallback.h',
             'dom/MutationObserver.cpp',
             'dom/MutationObserver.h',
             'dom/MutationObserverInterestGroup.cpp',
             'dom/MutationObserverInterestGroup.h',
@@ skipping 1332 matching lines @@
             'html/parser/HTMLParserThreadTest.cpp',
             'html/track/vtt/BufferedLineReaderTest.cpp',
             'html/track/vtt/VTTScannerTest.cpp',
             'rendering/RenderOverflowTest.cpp',
             'rendering/shapes/BoxShapeTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
         ],
     }
 }