Implement verification of context separation.

Implement verification of context separation.

This adds the --verify-global-context-separation flag which can be used
to verify that no code object embeds pointers to more than one global
context after a full GC. It uses an object visitor that just performs
shallow traversal of the object graph spanned by one code object, and
breaks at points where application objects are encountered. So it will
not trip on cross-context leaks introduced by the application itself.

R=verwaest@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10830049

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12224 ce2b1a6d-e550-0410-aec6-3dcde31c8c00