Add adoptRefCountedGarbageCollected method and transition version thereof.

When returning a raw pointer instead of a PassRefPtr to something that is
RefCountedGarbageCollected we need to make sure that we 'adopt' the ref.

BUG=

Review URL: https://codereview.chromium.org/152673003

git-svn-id: svn://svn.chromium.org/blink/trunk@166385 bbb929c8-8fbe-4397-9dbb-9b2b20218538