Port cdc43bc5

Original review: https://codereview.chromium.org/1133163005/

Original commit message:
Enable clang's shorten-64-to-32 warning flag on ARM64, and fix the warnings
that arise.

BUG=

Review URL: https://codereview.chromium.org/1182193004

Cr-Commit-Position: refs/heads/master@{#29024}