Text::createTextRenderer() should return a RenderPtr.
<https://webkit.org/b/126292>

Make createTextRenderer() return a RenderPtr and remove one manual
destroy() call. Also, since it should always return a valid object,
I turned a null check into an assertion instead.

Reviewed by Antti Koivisto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@161143 268f45cc-cd09-0410-ab3c-d52691b4dbfc