Web Animations API: Avoid crash when timeline outlives document

Ensure that the timeline is detached from a document during document
destruction. The timeline should actually keep the document alive but
we're deferring such lifetime issues until a future iteration.

BUG=343582

Review URL: https://codereview.chromium.org/165503002

git-svn-id: svn://svn.chromium.org/blink/trunk@167157 bbb929c8-8fbe-4397-9dbb-9b2b20218538