Prepare SVGSMILElement for oilpan.

In oilpan builds, |m_syncBaseDependents| and |m_syncBase| are changed to weak references from manual detach in d-tors.

|m_syncBase| is changed from Element* -> SVGSMILElement* as its only valid value is SVGSMILElement*.

BUG=357163

Review URL: https://codereview.chromium.org/305783004

git-svn-id: svn://svn.chromium.org/blink/trunk@175088 bbb929c8-8fbe-4397-9dbb-9b2b20218538