Web Animations: Break reference cycle keeping Elements alive

We currently have a couple of reference cycles, basically stemming from the
strong pointer Animation->Element. As a result, we will not be able to free
Elements when they have current or in effect Animations, even if there are
no longer any references around (or we've navigated to a separate page!)

This is a temporary patch (until Element is moved to Oilpan) to break the
reference cycles that we have (see referenced bug). We simply change the
Animation->Element pointer to a raw pointer, and clean it upon Element
destruction. Note that this is somewhat incorrect behaviour, since
Animations should keep their target alive if they are still reachable,
but we probably can't easily do this without leaking before we have Oilpan.

BUG=362404

Review URL: https://codereview.chromium.org/236833002

git-svn-id: svn://svn.chromium.org/blink/trunk@171537 bbb929c8-8fbe-4397-9dbb-9b2b20218538