Fix potential assertion failure in ConvertCase.

IsOneByteRepresentationUnderneath requires the string to be flat, which is not
guaranteed in this case.  Using IsOneByteRepresentation is sufficient here.

R=mvstanton@chromium.org
BUG=

Review URL: https://codereview.chromium.org/62493007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17620 ce2b1a6d-e550-0410-aec6-3dcde31c8c00