Web Animations CSS: Allow positive start-delay animations on the compositor.

(Depends on a fix to the Chromium compositor to properly support negative time offsets https://codereview.chromium.org/95603003 .)

As we now can directly forward the startDelay into timeOffset we no longer adjust the iterationCount.

BUG=324670
DEPENDS=95603003

Review URL: https://codereview.chromium.org/96193002

git-svn-id: svn://svn.chromium.org/blink/trunk@162959 bbb929c8-8fbe-4397-9dbb-9b2b20218538