Updated syncer_helper to use children()[] and children().size()
instead. Updated relevant code to use size_t instead of int/uint64_t.

Chromium changes:

https://chromium.googlesource.com/chromium/src/+/5174590b09badcd86c14d322266039782e94a532

commit 5174590b09badcd86c14d322266039782e94a532
Author: Peter Kasting <pkasting@chromium.org>
Date:   Fri Jun 28 21:54:55 2019 +0000

    Remove TreeNode::GetChild().

    Bug: 956419

https://chromium.googlesource.com/chromium/src/+/b0d21f299ba5fd0c51c26f1f440fb1a006fc4753

commit b0d21f299ba5fd0c51c26f1f440fb1a006fc4753
Author: Peter Kasting <pkasting@chromium.org>
Date:   Tue Jun 25 00:26:16 2019 +0000

    Remove instances of TreeNode::child_count().

    Bug: 956419