Devtools Animations: Fix current time slider issues

When a CSS transition is finished, setting current time has no effect.
With other animations that finish, a jump is observed on the slider.
This change fixes these issues and also pauses the animation while the
slider is being dragged to avoid the animation and slider appearing
jittery.

BUG=434616

Review URL: https://codereview.chromium.org/742673003

git-svn-id: svn://svn.chromium.org/blink/trunk@185744 bbb929c8-8fbe-4397-9dbb-9b2b20218538