Web Animations: Refactor AnimatableLength's unit type enum

This patch is preparation work for moving AnimatableLength::LengthUnitType
into CSSPrimitiveValue. This patch renames NumberUnitType to LengthUnitType
and associated AnimatableLength members.

Review URL: https://codereview.chromium.org/227433002

git-svn-id: svn://svn.chromium.org/blink/trunk@171096 bbb929c8-8fbe-4397-9dbb-9b2b20218538