Workaround for http://crbug.com/9746:
- Added special cutouts if a Vector has NULL data, which will now happen
if an external string's resource has been deleted.
- Added an verification phase before old gen GC to verify that all real
entries in the SymbolTable are valid symbols.
- Added test that verifies the correct behaviour of the workaround.
Review URL: http://codereview.chromium.org/66011
git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1689
ce2b1a6d-e550-0410-aec6-
3dcde31c8c00