Web Animations API: Interpolate into reference rather than returning new value.

This is a re-work of https://codereview.chromium.org/238313003/

Review URL: https://codereview.chromium.org/712143003

git-svn-id: svn://svn.chromium.org/blink/trunk@185088 bbb929c8-8fbe-4397-9dbb-9b2b20218538