Minor cleanups in AnimatableValue code

A few minor cleanups to the AnimatableValue classes. Basically this
makes the interface more like the rest of Blink.

- Make isNeutral non-virtual and add isNumber, isUnknown (note that we
    currently can only detect subclass type with dynamic_cast since
    m_type is protected)
- Add toAnimatableNumber
- Make AnimatableValue::m_type and AnimatableUnknown::m_value const

BUG=

Review URL: https://chromiumcodereview.appspot.com/22274006

git-svn-id: svn://svn.chromium.org/blink/trunk@155595 bbb929c8-8fbe-4397-9dbb-9b2b20218538