Add ShadowIncludingTreeOrderTraversal::InclusiveDescendantsOf()

Just a utility function to go along with the other such methods
in other traversal methods.

I also removed one unneeded `protected:` from NodeRareData.

No functionality change here.

Change-Id: Idadfb11fb2c19d7ddbc762b5d8473c993e7230a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5336597


Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Di Zhang <dizhangg@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267440}