Oilpan: improve ScriptPromiseProperty<>'s argument types.

The ScriptPromiseProperty<> template will instantiate three fields holding
references to objects. In order to ensure lifetime safety, supply better
transition types where it is used.

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

Review URL: https://codereview.chromium.org/393773004

git-svn-id: svn://svn.chromium.org/blink/trunk@178278 bbb929c8-8fbe-4397-9dbb-9b2b20218538