resource to NULL when removing it from the symbol table.  This makes
sure that the debugger can recognize the external string as being
"deleted".  Now, whenever an external resource is deleted, the
resource pointer is set to NULL.

This is really a workaround of a debugger problem.  We need to make
sure that the debugger only finds scripts in the heap that are
actually live.
Review URL: http://codereview.chromium.org/69029

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1734 ce2b1a6d-e550-0410-aec6-3dcde31c8c00