Add liveness tests to JSC API entry points
https://bugs.webkit.org/show_bug.cgi?id=114318

Reviewed by Geoffrey Garen.

Add simple checks for the existence of a method table on any
JSCells passed across the API.  This in turn forces a structure
validity test.

* API/APICast.h:
(toJS):
(toJSForGC):
(unsafeToJS):
* API/JSObjectRef.cpp:
(JSObjectGetPrivate):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148073 268f45cc-cd09-0410-ab3c-d52691b4dbfc