[V8] Generalize external object resources

[V8] Generalize external object resources

V8 was already able to manage and finalize an external string
resource.  This change generalizes that mechanism to handle a
single generic external resource - a v8::Object::ExternalResource
derived instance - on normal JSObject's.

This is useful for mapping C++ objects to JS objects where the
C++ object's memory is effectively owned by the JS Object, and
thus needs to destroyed when the JS Object is garbage collected.
The V8 mailing list suggests using a weak persistent handle for
this purpose, but that seems to incur a fairly massive performance
penalty for short lived objects as weak persistent handle callbacks
are not called until the object has been promoted into the old
object space.

Change-Id: I3d7b5fe4ca6e37d49c92f3314b80d42bd4c84fcd
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>