Add a child renderer iterator.
<https://webkit.org/b/123662>

Introduce an ElementIterator-style iterator for renderers and put
it to use in a childrenOfType() implementation.

It's used just like the Element iterators:

auto sections = childrenOfType<RenderTableSection>(*this);
for (auto section = sections.begin(), section = sections.end(); section != sections.end(); ++section)
    section->thisOrThat();

Just like the DOM counterpart, it relies on a templatized helper:

    bool isRendererOfType<T>(const RenderObject&)

This patch puts the iterator to use in a couple of random places.

Reviewed by Antti Koivisto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@158495 268f45cc-cd09-0410-ab3c-d52691b4dbfc