Prepare SMILTimeContainer for oilpan

This prepares |SMILTimeContainer::m_scheduledAnimations| for oilpan.
- Vector<SVGSMILElement*> is changed to LinkedHashSet, as oilpan do not support having weak members as vector element.
- Entries pointing to disposing elements are automatically rewritten by oilpan weak processing. They are explicitly removed at |updateAnimations()|

BUG=357163

Review URL: https://codereview.chromium.org/304453002

git-svn-id: svn://svn.chromium.org/blink/trunk@174933 bbb929c8-8fbe-4397-9dbb-9b2b20218538