Web Animations: Expose timeline currentTime in IDL

AnimationTimeline has currentTime of type double - the time value for
this timeline.

Web Animations spec, section 5.1 The AnimationTimeline interface
http://dev.w3.org/fxtf/web-animations/#the-animationtimeline-interface


BUG=357432

Review URL: https://codereview.chromium.org/215043007

git-svn-id: svn://svn.chromium.org/blink/trunk@170513 bbb929c8-8fbe-4397-9dbb-9b2b20218538