SVG: Remove ElementDependenciesMap.

Before this patch, SVGElement references were managed in
|SVGDocumentExtentions::m_elementReferencesMap|. This was implemented as
a map<SVGElement*, set<SVGElement*>>, and was hard to manage.
In particular, clearing out map entry with empty set<> is hard in oilpan.

This patch moves this bookkeeping into SVGElementRareData.
We introduce two set<SVGElement*> to manage incoming/outgoing references.

This patch keeps the current reference management interfaces in
SVGDocumentExtentions class. The management methods are to be moved to
SVGElement in a follow up CL.

BUG=378670

Review URL: https://codereview.chromium.org/338833007

git-svn-id: svn://svn.chromium.org/blink/trunk@176803 bbb929c8-8fbe-4397-9dbb-9b2b20218538