Oilpan: Trace SVGFontFaceElement::m_fontElement.

m_fontElement is a parent element. It's ok to use a strong reference.

BUG=357163

Review URL: https://codereview.chromium.org/338213004

git-svn-id: svn://svn.chromium.org/blink/trunk@176280 bbb929c8-8fbe-4397-9dbb-9b2b20218538