Start the migration of passwords from the Keychain.

Design doc: https://docs.google.com/a/google.com/document/d/1A8ZG16bLuUH1u21K0GoABKz_wpz1kchXMnMlpmq_ecA/edit?usp=sharing

This is a reland of https://codereview.chromium.org/1213043003/

BUG=466638
TBR=isherman@chromium.org

Review URL: https://codereview.chromium.org/1226303003

Cr-Commit-Position: refs/heads/master@{#338010}
17 files changed