Add an owning smart pointer for RenderObjects and start using it.

Add an owning smart pointer for RenderObjects and start using it.
<https://webkit.org/b/126251>

This patch adds a RenderPtr pointer, essentially an OwnPtr for
RenderObjects. The difference is that RenderPtr destroys the object
by calling destroy() on it.

This is necessary to implement the willBeDestroyed() mechanism in
RenderObject that notifies renderers just before they are about to
be deleted, while they can still do tree traversal, etc.

I also added a make_unique-alike helper so you can write:

    auto renderer = createRenderObject<RenderImage>(...);

Put it all to use by making ContentData::createRenderer() return
RenderPtr<RenderObject> instead of raw RenderObject*.

Reviewed by Antti Koivisto.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@161115 268f45cc-cd09-0410-ab3c-d52691b4dbfc