Revert of Oilpan: Trace CSSFontFaceSrcValue::m_svgFontFaceElement. (https://codereview.chromium.org/333403004/)

Reason for revert:
This patch leaks svg-font-face-leak-document test

http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=svg/svg-font-face-leak-document.html

Original issue's description:
> Oilpan: Trace CSSFontFaceSrcValue::m_svgFontFaceElement.
> 
> BUG=357163
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176270

BUG=357163

Review URL: https://codereview.chromium.org/337703004

git-svn-id: svn://svn.chromium.org/blink/trunk@176299 bbb929c8-8fbe-4397-9dbb-9b2b20218538