Prepare |SVGDocumentExtensions::m_relativeLengthSVGRoots| for oilpan

|SVGDocumentExtensions::m_relativeLengthSVGRoots| is made a HashSet<Member<SVGSVGELement> > in oilpan.
This is valid in oilpan, because document and <svg> elements are guaranteed to die at the same time. When a <svg> element is removed from the document, |SVGSVGElement::removedFrom| would remove itself from the set.

BUG=357163

Review URL: https://codereview.chromium.org/323803002

git-svn-id: svn://svn.chromium.org/blink/trunk@175865 bbb929c8-8fbe-4397-9dbb-9b2b20218538