Oilpan: Move ScriptFunction to oilpan's heap

This CL fixes crashes in oilpan builds introduced by r180902.

The problem was that RespondWithObserver::ThenFunction and WaitUntilObserver::ThenFunction
hold a persistent handle, and the persistent handle remains until the end of the worker thread
execution if the ThenFunction is not resolved nor rejected by user's script.
Then the final Oilpan's GC his the empty ASSERT.

This CL fixes the issue by moving the ThenFunction to oilpan's heap and replaces
the persitent handle with a Member.

BUG=340522

Review URL: https://codereview.chromium.org/490103005

git-svn-id: svn://svn.chromium.org/blink/trunk@180952 bbb929c8-8fbe-4397-9dbb-9b2b20218538