The render descendant iterator should work with const objects

• Component: New Bugs
• Status: NEW ---

[Gabriel Nava Marino]

Currently the following code in RenderBlockFlow.cpp:
```
for (auto& block : descendantsOfType<RenderBlock>(*this))
    ...
```

Will lead to the following compile-time error:
```
./rendering/RenderIterator.h:131:12: error: no matching function for call to 'nextAncestorSibling'
    return nextAncestorSibling(current, stayWithin);
```

It was mentioned this is due to a "constness issue; const_cast makes it work. We should teach the descendant iterator to work with const objects. childrenOfType works fine so it should be relatively easy."

[Radar WebKit Bug Importer]

<rdar://problem/86787920>