Always make a copy of a string when adding it to StringsStorage

Otherwise the string passed as const char* may be disposed and we will end up with a dangling pointer.

Also changed StringsStorage::GetCopy so that a copy is not created if the string is already in the cache.

BUG=None
R=alph@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/27627006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17260 ce2b1a6d-e550-0410-aec6-3dcde31c8c00