Invoke RefPtr::release in GenerateFunctionCallString where possible

Previously, we'd sometimes end up with two refs (by passing a RefPtr
where a PassRefPtr is expected) and sometimes with the caller seeming to have
ownership where it actually didn't (by invoking .get() instead of .release()).

The new logic now passes .release() wherever a PassRefPtr is expected, since
the calling code (the bindings) won't do anything else with that pointer anyway.

This may pave the way toward making Callbacks OwnPtrs, but I wouldn't bet on it.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/85473006

git-svn-id: svn://svn.chromium.org/blink/trunk@162649 bbb929c8-8fbe-4397-9dbb-9b2b20218538