Make SVGProperty RefCountedGarbageCollected to prepare for oilpan.

SVGProperty will be GarbageCollected at the end, but I'm making this RefCountedGarbageCollected first so we can work on this incrementally.

BUG=357163

Review URL: https://codereview.chromium.org/282753006

git-svn-id: svn://svn.chromium.org/blink/trunk@173967 bbb929c8-8fbe-4397-9dbb-9b2b20218538