Introduce DEFINE_ANIMATABLE_VALUE_TYPE_CASTS to replace manual toFoo() for animatable value.

As a step to let toFoo() use DEFINE_TYPE_CASTS, this cl replace toFoo() for animatable value.
with DEFINE_ANIMATABLE_VALUE_TYPE_CASTS. To use DEFINE_TYPE_CASTS generates plenty
more helper functions which can detect bad type cast.

BUG=309516

Review URL: https://codereview.chromium.org/29543005

git-svn-id: svn://svn.chromium.org/blink/trunk@160151 bbb929c8-8fbe-4397-9dbb-9b2b20218538