Web Animations: Introduce ActiveAnimations and AnimationStack

Source/core/core.gypi

 {
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             'css/CSSCharsetRule.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSHostRule.idl',
             'css/CSSImportRule.idl',
             'css/CSSMediaRule.idl',
@@ skipping 523 matching lines @@
             'accessibility/AccessibilityTableRow.h',
             'animation/AnimatableNeutral.h',
             'animation/AnimatableNumber.cpp',
             'animation/AnimatableNumber.h',
             'animation/AnimatableUnknown.h',
             'animation/AnimatableValue.cpp',
             'animation/AnimatableValue.h',
             'animation/Animation.cpp',
             'animation/Animation.h',
             'animation/AnimationEffect.h',
+            'animation/AnimationStack.cpp',
+            'animation/AnimationStack.h',
             'animation/DocumentTimeline.cpp',
             'animation/DocumentTimeline.h',
             'animation/KeyframeAnimationEffect.cpp',
             'animation/KeyframeAnimationEffect.h',
             'animation/Player.cpp',
             'animation/Player.h',
             'animation/TimedItem.cpp',
             'animation/TimedItem.h',
             'animation/TimedItemCalculations.h',
             'animation/Timing.h',
@@ skipping 3217 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++"'
               }],
          ],
     }
 }