Web Animations: Don't set compositor pending when document has been detached

When there are multiple documents it is possible for an animation's
timeline to become detached. The existing assertions were not valid in this
case.

BUG=492061

Review URL: https://codereview.chromium.org/1360043002

git-svn-id: svn://svn.chromium.org/blink/trunk@202666 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed