[Sync:USS] Support decryption for explicit passphrase

After receiving the Nigori with explicit passphrase type bridge
requests passphrase via OnPassphraseRequired() observer's method.
Calling OnPassphraseRequired() should make UI ask for passphrase and
calling SetDecryptionPassprase() once it is provided. UI part is
responsible for verifying passphrase by using pending keys exposed by
OnPassphraseRequired().

This CL doesn't contain logic for SCRYPT key derivation method and
triggering Nigori rewrite in case we have additional encryption keys
locally.

Bug: 922900
Change-Id: I59d9fb48773232c68631e8c1ce5cd7863b638fa3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1621911
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663014}
3 files changed