Web Animations: Add Interpolation class.

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 577 matching lines @@
             'animation/CompositorAnimationsImpl.h',
             'animation/DocumentAnimation.h',
             'animation/DocumentAnimations.cpp',
             'animation/DocumentAnimations.h',
             'animation/DocumentTimeline.cpp',
             'animation/DocumentTimeline.h',
             'animation/ElementAnimation.cpp',
             'animation/ElementAnimation.h',
             'animation/InertAnimation.cpp',
             'animation/InertAnimation.h',
+            'animation/Interpolation.cpp',
+            'animation/Interpolation.h',
             'animation/InterpolableValue.cpp',
             'animation/InterpolableValue.h',
             'animation/KeyframeEffectModel.cpp',
             'animation/KeyframeEffectModel.h',
             'animation/Player.cpp',
             'animation/Player.h',
             'animation/TimedItem.cpp',
             'animation/TimedItem.h',
             'animation/TimedItemCalculations.h',
             'animation/TimedItemTiming.cpp',
@@ skipping 2679 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',
         ],
     }
 }