We're probably not going to use it again, and I don't want to make the
effort of rewriting it in Id867b3005ebc46906d6df852a525fcaec9e6b19b.
Change-Id: I0b02533f7c9b8c1b0df271e03a74063f123d0dff
* Pass rootNode to the constructor
* Rename getters to match CommentItem/HeadingItem/ThreadItem
value classes.
* Always build the thread tree so CommentItem's always have
and ID and replies/parent.
Change-Id: I508be9534de59016ff806e3d84edcbb1c76cb0c6