Set DocumentTimeline's current time when setting its zero time

When CSS Animations are created, they are added to the DocumentTimeline, which
immediately updates their timing parameters. This occurs before
DocumentTimeline::serviceAnimations() is called, so the DocumentTimeline's
current time must be set as soon as the zero time is available.

BUG=240653
R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/22882006

git-svn-id: svn://svn.chromium.org/blink/trunk@156291 bbb929c8-8fbe-4397-9dbb-9b2b20218538