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Once explicit passphrase is known by the client, it should be persisted
somewhere to not ask user to provide it on every Chrome startup. While
for USS implementation we're going to use different way for key
persistence, we still store it in preferences, because:

1. It allows to start experiments without complete implementation of
new persistence approach.

2. It allows to disable USS Nigori implementation and continue using
Directory implementation without asking user to enter passphrase again.

Bug: 922900
Change-Id: I2295872752b23901c7f0915b1d156e28dc54d961
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1709326
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680048}