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
Change-Id: I2c4d1700571a7cd00cef926beb525661c7073e0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636254
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Tatiana Gornak <melandory@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Cathy Li <chili@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666538}
106 files changed