Fix v8 Windows GN build.

The paths for some of the Windows-specific files was incorrect.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/402693002

Patch from Brett Wilson <brettw@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22455 ce2b1a6d-e550-0410-aec6-3dcde31c8c00