Cache element indices for :nth-child and :nth-last-child selectors.

Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/AnimationEffect.idl',
             'animation/AnimationNode.idl',
@@ skipping 2251 matching lines @@
             'dom/NodeListsNodeData.h',
             'dom/NodeRareData.cpp',
             'dom/NodeRareData.h',
             'dom/NodeRenderingTraversal.cpp',
             'dom/NodeRenderingTraversal.h',
             'dom/NodeTraversal.cpp',
             'dom/NodeTraversal.h',
             'dom/NodeWithIndex.h',
             'dom/NonDocumentTypeChildNode.h',
             'dom/NonElementParentNode.h',
+            'dom/NthIndexCache.cpp',
+            'dom/NthIndexCache.h',
             'dom/ParentNode.h',
             'dom/PendingScript.cpp',
             'dom/Position.cpp',
             'dom/PositionIterator.cpp',
             'dom/PositionIterator.h',
             'dom/PresentationAttributeStyle.cpp',
             'dom/PresentationAttributeStyle.h',
             'dom/ProcessingInstruction.cpp',
             'dom/ProcessingInstruction.h',
             'dom/PseudoElement.cpp',
@@ skipping 1459 matching lines @@
             'css/parser/MediaConditionTest.cpp',
             'css/parser/SizesAttributeParserTest.cpp',
             'css/parser/SizesCalcParserTest.cpp',
             'css/resolver/FontBuilderTest.cpp',
             'dom/ActiveDOMObjectTest.cpp',
             'dom/CrossThreadTaskTest.cpp',
             'dom/DOMImplementationTest.cpp',
             'dom/DocumentMarkerControllerTest.cpp',
             'dom/DocumentTest.cpp',
             'dom/MainThreadTaskRunnerTest.cpp',
+            'dom/NthIndexCacheTest.cpp',
             'dom/RangeTest.cpp',
             'dom/ScriptRunnerTest.cpp',
             'dom/TreeScopeTest.cpp',
             'dom/shadow/ComposedTreeTraversalTest.cpp',
             'editing/CompositionUnderlineRangeFilterTest.cpp',
             'editing/FrameSelectionTest.cpp',
             'editing/InputMethodControllerTest.cpp',
             'editing/iterators/TextIteratorTest.cpp',
             'editing/SurroundingTextTest.cpp',
             'editing/VisibleSelectionTest.cpp',
@@ skipping 54 matching lines @@
             'paint/LayerClipRecorderTest.cpp',
             'paint/LayoutObjectDrawingRecorderTest.cpp',
             'paint/TextPainterTest.cpp',
             'streams/ReadableStreamReaderTest.cpp',
             'streams/ReadableStreamTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }