Avoid duplication of StringAddFlags in the platform specific code stubs header files.
Fix the inverted flag logic, replacing it with a scheme that is easier to understand.

Depends on: https://codereview.chromium.org/19541003

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/19492006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15775 ce2b1a6d-e550-0410-aec6-3dcde31c8c00