Fix potential assertion failure in ConvertCase.
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: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@17620 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh