wstring: remove wstring version of SplitString

Retry of r84336.

BUG=23581

Review URL: http://codereview.chromium.org/6930047

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@84355 0039d316-1c4b-4281-b951-d872f2087c98
const Background* Label::GetMouseOverBackground() const {
  return mouse_over_background_.get();
}

const Background* Label::GetMouseOverBackground() const {
  return mouse_over_background_.get();
}
