Convert the boolean param of Image::startAnimation() to an enum

This patch is a merge of wkrev.com/167356 by Simon Fraser (smfr). The
enum parameter is more readable.

BUG=364721

Review URL: https://codereview.chromium.org/242193002

git-svn-id: svn://svn.chromium.org/blink/trunk@171912 bbb929c8-8fbe-4397-9dbb-9b2b20218538