Web Animations: Zero time of the document timeline should correspond with navigationStart

See: http://dev.w3.org/fxtf/web-animations/#dfn-zero-time

Review URL: https://codereview.chromium.org/551253002

git-svn-id: svn://svn.chromium.org/blink/trunk@181608 bbb929c8-8fbe-4397-9dbb-9b2b20218538