Oilpan: Animation::m_target shouldn't become a stale pointer

This is a CL to fix a regression caused by r171537.

Currently Animation::notifyElementDestroyed() is called in the destructor of ActiveAnimations. This is too late because ActiveAnimations is on heap and the destructor is delayed to the next GC. Consequently, Animation::m_target can become a stale poiter and cause crash in oilpan builds.

To fix the issue, this CL changes code so that Animation::notifyElementDestroyed() is called in the destructor of Element.

BUG=340522

Review URL: https://codereview.chromium.org/242773003

git-svn-id: svn://svn.chromium.org/blink/trunk@172009 bbb929c8-8fbe-4397-9dbb-9b2b20218538