Oilpan: fix ref counting of derived SVG property objects.

Following r183582, RefCountedGarbageCollected<T>::m_refCount is left as
0. This is incompatible with the derived SVG objects which have a base
object (SVGPropertyBase) that's a RefCountedGarbageCollected<> object,
but the derived objects are all exposed as if they were deriving from
a RefCounted<T> base.

Adjust the ref count upon construction of SVGPropertyBase to provide
that illusion.

TBR=haraken,oilpan-reviews
BUG=340522
NOTRY=true

Review URL: https://codereview.chromium.org/650033002

git-svn-id: svn://svn.chromium.org/blink/trunk@183586 bbb929c8-8fbe-4397-9dbb-9b2b20218538