Heap-use-after-free in WebCore::RenderObject::childAt

Bug: 11676314

This is a cherry pick of https://codereview.chromium.org/30663003.
Note that it is a cherry-pick of the code only (not the layout
tests). The cherry pick had one minor modification from a reference
to a pointer type.

Change-Id: I4aa902aaa04970eba548ad2eac6d2a288f60bc49
1 file changed