Element's renderer factory should return RenderPtrs.

Element's renderer factory should return RenderPtrs.
<https://webkit.org/b/126318>

Rename Element::createRenderer() to createElementRenderer() and have
it return RenderPtr<RenderElement>. Propagate signature until it
builds again.

We leakPtr() the renderer at two call sites when handing things over
to raw pointer API. This'll get tidied up in subsequent patches.

Reviewed by Sam Weinig.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@161181 268f45cc-cd09-0410-ab3c-d52691b4dbfc