Web Animations CSS: Support animation of background-position and background-image

Source/core/core.gypi

 {
     'variables': {
         'core_idl_files': [
             'css/CSS.idl',
             'css/CSSCharsetRule.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSHostRule.idl',
             'css/CSSImportRule.idl',
             'css/CSSKeyframeRule.idl',
@@ skipping 520 matching lines @@
             'animation/AnimatableDouble.h',
             'animation/AnimatableImage.cpp',
             'animation/AnimatableImage.h',
             'animation/AnimatableLength.cpp',
             'animation/AnimatableLength.h',
             'animation/AnimatableLengthBox.cpp',
             'animation/AnimatableLengthBox.h',
             'animation/AnimatableLengthSize.cpp',
             'animation/AnimatableLengthSize.h',
             'animation/AnimatableNeutral.h',
+            'animation/AnimatableRepeatable.cpp',
+            'animation/AnimatableRepeatable.h',
             'animation/AnimatableSVGLength.cpp',
             'animation/AnimatableSVGLength.h',
             'animation/AnimatableShapeValue.cpp',
             'animation/AnimatableShapeValue.h',
             'animation/AnimatableSVGPaint.cpp',
             'animation/AnimatableSVGPaint.h',
             'animation/AnimatableTransform.cpp',
             'animation/AnimatableTransform.h',
             'animation/AnimatableUnknown.h',
             'animation/AnimatableValue.cpp',
@@ skipping 3099 matching lines @@
                 'bison_exe': 'bison',
                 # We specify a preprocess so it happens locally and won't get
                 # distributed to goma.
                 # FIXME: /usr/bin/gcc won't exist on OSX forever. We want to
                 # use /usr/bin/clang once we require Xcode 4.x.
                 'preprocessor': '--preprocessor "/usr/bin/gcc -E -P -x c++"'
               }],
          ],
     }
 }