Remove special handling of RenderLineBreak in container/containingBlock.
Special handling of RenderLineBreak in container/containingBlock is required to handle the case where the line break is out-of-flow. This can't be supported as part of the normal render tree out-of-flow treatment, since for out-of-flow to work the line break would need to be represented as a RenderBox, but RenderLineBreak can't be a block type.
This bug is intended as a reminder to remove the need for the special treatment. Note that LFC has better support for out-of-flow line breaks.
<rdar://problem/125168158>