Web Animations: Don't mark players as outdated upon currentTime access

Accessing currentTime on a Player logically doesn't mark it as outdated,
although currently it does as it updates time to match the timeline.
Removing this behaviour makes some of the logic in serviceAnimations and
Player::update slightly nicer. We also tweak the logic somewhat so as to
remove a source of rounding errors in timing calculations.

BUG=334926

Review URL: https://codereview.chromium.org/172003002

git-svn-id: svn://svn.chromium.org/blink/trunk@168628 bbb929c8-8fbe-4397-9dbb-9b2b20218538