Implement RefCountedGarbageCollected class for the blink gc infrastructure.

RefCountedGarbageCollected is used to incrementally move objects from
being reference counted to being managed by the Blink garbage collection
infrastructure.

R=erik.corry@gmail.com, haraken@chromium.org, wibling@chromium.org
BUG=

Review URL: https://codereview.chromium.org/133593002

git-svn-id: svn://svn.chromium.org/blink/trunk@164867 bbb929c8-8fbe-4397-9dbb-9b2b20218538