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

Source/platform/blink_platform.gypi

 {
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.cpp',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
       'Clock.h',
       'ColorChooser.cpp',
@@ skipping 888 matching lines @@
       'text/SegmentedStringTest.cpp',
       'text/UnicodeUtilitiesTest.cpp',
       'weborigin/DatabaseIdentifierTest.cpp',
       'weborigin/KURLTest.cpp',
       'weborigin/OriginAccessEntryTest.cpp',
       'weborigin/SecurityOriginTest.cpp',
       'weborigin/SecurityPolicyTest.cpp',
     ],
     # NOTE: these are legacy unit tests, do not add more!
     'platform_web_unittest_files': [
-      'animation/TimingFunctionTestHelper.cpp',
-      'animation/TimingFunctionTestHelper.h',
-      'animation/TimingFunctionTestHelperTest.cpp',
+      'animation/TimingFunctionTest.cpp',
       'graphics/DeferredImageDecoderTest.cpp',
       'graphics/ImageDecodingStoreTest.cpp',
       'graphics/ImageFrameGeneratorTest.cpp',
       'graphics/test/MockImageDecoder.h',
     ],
   },
 }