TEST=MSVS 2008 no longer generates warning C4251 for ExternalAsciiStringResourceImpl

Review URL: http://codereview.chromium.org/8056019

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9462 ce2b1a6d-e550-0410-aec6-3dcde31c8c00