Oilpan: pull SVGPropertyBase off the heap.

Having the SVG property base class derive from RefCountedGarbageCollected,
but its derived classes all be created and kept as ref-counted pointers
doesn't bring any benefits.

Revert back to deriving from RefCounted<> until the entire SVG property
hierarchy can be moved to the Oilpan heap, and ref-counts be avoided
altogether for these objects.

R=haraken
BUG=340522

Review URL: https://codereview.chromium.org/686163002

git-svn-id: svn://svn.chromium.org/blink/trunk@184600 bbb929c8-8fbe-4397-9dbb-9b2b20218538