Web Animation API: Add use counter for Animation constructors

DEPENDS ON: https://codereview.chromium.org/198693002/

BUG=352493,352490

Review URL: https://codereview.chromium.org/199743003

git-svn-id: svn://svn.chromium.org/blink/trunk@169721 bbb929c8-8fbe-4397-9dbb-9b2b20218538