Web Animations API: Change API time values from seconds to milliseconds

As discussed on blink-dev[1] and public-fx[2].

[1] https://groups.google.com/a/chromium.org/d/msg/blink-dev/SWxBI0uPvUE/iZCNwoN3nKEJ
[2] http://lists.w3.org/Archives/Public/public-fx/2014AprJun/0014.html

BUG=364347

Review URL: https://codereview.chromium.org/240493006

git-svn-id: svn://svn.chromium.org/blink/trunk@172332 bbb929c8-8fbe-4397-9dbb-9b2b20218538