Web Animations API: Implement step-middle and steps(x, middle) timing functions.

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 3233 matching lines @@
             'css/CSSTestHelper.cpp',
             'css/CSSTestHelper.h',
             'css/CSSValueTestHelper.h',
             'css/CSSParserValuesTest.cpp',
             'css/CSSSelectorTest.cpp',
             'css/CSSValueTestHelper.h',
             'css/DragUpdateTest.cpp',
             'css/HoverUpdateTest.cpp',
             'css/RuleSetTest.cpp',
             'dom/DOMImplementationTest.cpp',
+            'css/parser/BisonCSSParserTest.cpp',
             'dom/DocumentMarkerControllerTest.cpp',
             'dom/DocumentTest.cpp',
             'dom/MainThreadTaskRunnerTest.cpp',
             'editing/TextIteratorTest.cpp',
             'fetch/CachingCorrectnessTest.cpp',
             'fetch/ImageResourceTest.cpp',
             'fetch/MemoryCacheTest.cpp',
             'fetch/RawResourceTest.cpp',
             'fetch/ResourceFetcherTest.cpp',
             'frame/ImageBitmapTest.cpp',
             'html/HTMLDimensionTest.cpp',
             'html/LinkRelAttributeTest.cpp',
             'html/TimeRangesTest.cpp',
             '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',
         ],
     }
 }