Use WeakPtr for RenderTreePosition::m_nextSibling

Use WeakPtr for RenderTreePosition::m_nextSibling
https://bugs.webkit.org/show_bug.cgi?id=181363

Reviewed by Zalan Bujtas.

For safety. In most cases it is null and won't cause us to instantiate WeakReferences for
many new objects.

* rendering/updating/RenderTreePosition.cpp:
(WebCore::RenderTreePosition::computeNextSibling):
* rendering/updating/RenderTreePosition.h:
(WebCore::RenderTreePosition::RenderTreePosition):
(WebCore::RenderTreePosition::nextSibling const):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@226488 268f45cc-cd09-0410-ab3c-d52691b4dbfc