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Removes the last dubious API surface (size_t usage) in the core ABI and
documents the ABI as stable.

Bug: 822034,819046
Change-Id: I60438dd6613d45738514a92ce8c21777b859bfee
Reviewed-on: https://chromium-review.googlesource.com/1062932Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559348}