Expose TreeNode::children() publicly and use it to remove empty().

Since the public getter is const, this converts BookmarkModel (which was using a
non-const getter) to use direct member access, with a friend declaration.  Filed
bug 965314 about cleaning this up.

This also converts/comments the rest of tree_node_model.h in preparation for
removing child_count() and GetChild().

Bug: 956419
Change-Id: I28152e15ee380ada61a2668ab38bb53f25771b18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1621027
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661887}
31 files changed