GitLab

Add comments for struct members that were changed from size_t to uin32_t or uint64_t for IPC safety.

Also add a few checked_cast for places where we converted from size_t and we're sure that it's safe.

BUG=581409

Review URL: https://codereview.chromium.org/1684793003

Cr-Commit-Position: refs/heads/master@{#374788}