Chromium change:

https://chromium.googlesource.com/chromium/src/+/8cf4c23ebbd773240f207a8996941bd0343bd849

commit 8cf4c23ebbd773240f207a8996941bd0343bd849
Author: Peter Kasting <pkasting@chromium.org>
Date:   Thu Jun 6 00:38:30 2019 +0000

    Convert TreeNode{Model} Add/Remove functions to use size_t.

    This will make it more feasible to replace child_count() with children().size().

    This has a large number of followon effects.

    Bug: 956419