Delete weak JS objects on exit right away

Delete weak JS objects on exit right away

This reduces memory leaks on exit when the engine is destroyed
after exec() has finished.

Change-Id: I917d103966d55b4dd3ba4e986ff902e29d8fb0ac
Reviewed-by: Chris Adams <christopher.adams@nokia.com>
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>