After the page does navigations, garbage collection can still happen in
the old context. This commit changes to store references to remote objects
by _pages_, instead of by _WebContents_.
Previously we rely on the v8util.setDestructor to dereference the remote
objects in JavaScript, however as documented in V8, it is forbidden to
call V8 APIs in object's destructor (e.g. the weak callback), and doing
so would result in crashs.
This commit removes the JavaScript setDestructor method, and avoids
doing the dereference work with V8.
