Web Animations CSS: Avoid unnecessary allocations when interpolating AnimatableLengths

This patch pulls an optimisation between non calc lengths of the same type
into its own function. Other functions can now make use of the optimisation
without having to instantiate new objects for a call to create.

BUG=257591

Review URL: https://codereview.chromium.org/25713008

git-svn-id: svn://svn.chromium.org/blink/trunk@158884 bbb929c8-8fbe-4397-9dbb-9b2b20218538