Add support for bounds animations

cc/animation/animation_curve.cc

Index: cc/animation/animation_curve.cc
diff --git a/cc/animation/animation_curve.cc b/cc/animation/animation_curve.cc
index 9e8cae93538d0c32d89bade13d50165895b7a26d..b7c5891f7e436f29c75fa74e7b2803ef2b0e24a3 100644
--- a/cc/animation/animation_curve.cc
+++ b/cc/animation/animation_curve.cc
@@ -57,4 +57,13 @@ ScrollOffsetAnimationCurve* AnimationCurve::ToScrollOffsetAnimationCurve() {
   return static_cast<ScrollOffsetAnimationCurve*>(this);
 }
 
+const SizeAnimationCurve* AnimationCurve::ToSizeAnimationCurve() const {
+  DCHECK(Type() == AnimationCurve::SIZE);
+  return static_cast<const SizeAnimationCurve*>(this);
+}
+
+AnimationCurve::CurveType SizeAnimationCurve::Type() const {
+  return SIZE;
+}
+
 }  // namespace cc