Add composite() method to InterpolationType interface

Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/KeyframeEffect.idl',
             'animation/EffectModel.idl',
             'animation/AnimationEffectReadOnly.idl',
@@ skipping 912 matching lines @@
             'animation/StringKeyframe.cpp',
             'animation/StringKeyframe.h',
             'animation/StyleInterpolation.h',
             'animation/Timing.cpp',
             'animation/Timing.h',
             'animation/TimingCalculations.h',
             'animation/TimingInput.cpp',
             'animation/TimingInput.h',
             'animation/TransformSVGInterpolation.cpp',
             'animation/TransformSVGInterpolation.h',
+            'animation/UnderlyingValue.h',
             'animation/VisibilityStyleInterpolation.cpp',
             'animation/VisibilityStyleInterpolation.h',
             'animation/animatable/AnimatableClipPathOperation.cpp',
             'animation/animatable/AnimatableClipPathOperation.h',
             'animation/animatable/AnimatableColor.cpp',
             'animation/animatable/AnimatableColor.h',
             'animation/animatable/AnimatableDouble.cpp',
             'animation/animatable/AnimatableDouble.h',
             'animation/animatable/AnimatableDoubleAndBool.cpp',
             'animation/animatable/AnimatableDoubleAndBool.h',
@@ skipping 2917 matching lines @@
             'style/OutlineValueTest.cpp',
             'style/SVGComputedStyleTest.cpp',
             'svg/UnsafeSVGAttributeSanitizationTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }