Oilpan: make DOMWindowProperty a GC mixin.

As all objects deriving from DOMWindowProperty are on the heap, make
it a GC mixin and have it trace its Window reference. The registered
DOMWindowProperty instances on a LocalDOMWindow live at least as long
it.

The benefit is that we can remove its destructor. Deriving objects can
as a consequence remove their empty finalizers.

R=ager@chromium.org, haraken@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/552733003

git-svn-id: svn://svn.chromium.org/blink/trunk@181717 bbb929c8-8fbe-4397-9dbb-9b2b20218538