Remove unneeded AnimatableLength CSS code paths

AnimatableLength was originally designed with the expectation of being
used for responsive animations (e.g. 'em' units for font-size changes
or 'vw' units for window resizes), but these are now actually going to
be handled with the Interpolation system. As a result, most of the logic
in AnimatableLength is now unneeded, so this patch rewrites the class to
be much simpler.

I've also removed the PrintTo helper functions for AnimatableLength and
similar classes (instead of re-implementing them, since it'd likely end
up as wasted effort).

Review URL: https://codereview.chromium.org/268743005

git-svn-id: svn://svn.chromium.org/blink/trunk@173287 bbb929c8-8fbe-4397-9dbb-9b2b20218538