Web Animations API: Refactor IDL input conversion out of Animation

This change moves effect and timing conversion logic out of Animation
and into EffectInput and TimingInput respectively.

This refactor includes a minor change in behaviour:
Animations created on elements without an active document renderer will no
longer return null, instead the animation's effect will be null.
This behaviour still avoids calls to the CSS parser and style resolver if
the check fails.

Review URL: https://codereview.chromium.org/182063005

git-svn-id: svn://svn.chromium.org/blink/trunk@168575 bbb929c8-8fbe-4397-9dbb-9b2b20218538