Fix assert failure in AnimatableStrokeDasharrayList::equalTo()

Inherited methods were asserting AnimatableStrokeDasharrayList objects
were the parent class AnimatableValue type. Changed type check to allow
the subclass when up casting.
Added unittest case to test fix.

Review URL: https://codereview.chromium.org/27572003

git-svn-id: svn://svn.chromium.org/blink/trunk@160064 bbb929c8-8fbe-4397-9dbb-9b2b20218538