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This CL supports transitions from trusted vault passphrase to keystore
and custom passphrases. To avoid data corruptions by buggy clients the
bridge verifies that the remote keybag contains the last trusted vault
key, which was used as the default key. Since browser restart can
happen in between of receiving the remote update and decryption of the
new remote keybag, |last_trusted_vault_key| stored in NigoriLocalData.

Bug: 1020084
Change-Id: Ica654898a3499f599258bf9e65eb8d91580fcc2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087766
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747697}