Move static methods out of CSSLengthInterpolationType to LengthInterpolationFunctions

This change refactors the InterpolationValue logic out of
CSSLengthInterpolationType into its own file LengthInterpolationFunctions.
This allows other CSSInterpolationTypes to use Length functions without
importing unnecessary InterpolationType methods.

Review-Url: https://codereview.chromium.org/2283783002
Cr-Commit-Position: refs/heads/master@{#416877}
18 files changed