We want to decouple gr-diff from the comment thread element, but we
need to keep the comment thread group element because the grouping
depends on the viewMode and so on.
This means combining comments into threads should not be the
responsibility of the comment thread group element, because other
comment thread implementations use models that already come
threaded.
This is only the very first step:
* creation of the gr-diff-comment-thread elements also needs to move
out of gr-diff-comment-thread-group.
* gr-diff-builder.js is only a temporary home - eventually both
threading and rendering will move all the way out to gr-diff-host
Change-Id: Ifa3853aab619776609d42bc0d1422f4bdf598e3d
45365b9bc2