Prepare |SVGDocumentExtentsions::m_timeContainers| for oilpan

BUG=357163

Review URL: https://codereview.chromium.org/279403004

git-svn-id: svn://svn.chromium.org/blink/trunk@173964 bbb929c8-8fbe-4397-9dbb-9b2b20218538