Web Animations: Move Interpolation classes into subdirectory

Move Interpolation and its subclasses into distinct source files, in a new interpolation directory.

R=alancutter

Review URL: https://codereview.chromium.org/294603002

git-svn-id: svn://svn.chromium.org/blink/trunk@174629 bbb929c8-8fbe-4397-9dbb-9b2b20218538