Oilpan: pull SVGPropertyBase off the heap.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@184600 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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