Web Animations: Move TimedItem::activeDuration out of updateInheritedTime and add end time

We will require later that the end time of a TimedItem is calculated
before updating inherited time (for Player time calculations to get
the inherited time). This patch makes active duration able to be
retrieved straight away and adds end time. Note that the end time
function here doesn't take end delay into account as it isn't yet
implemented.

BUG=334927

Review URL: https://codereview.chromium.org/132693014

git-svn-id: svn://svn.chromium.org/blink/trunk@165559 bbb929c8-8fbe-4397-9dbb-9b2b20218538