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This Cl includes:
- Add a feature flag which is disabled by default.
- Include key creation utility to prevent key overwrites in KeychainPassword::GetPassword().
- Add tests for the above changes.
- Register the local state early from the main thread in os_crypt.

Bug: 791541
Change-Id: I2a664cd285fe73b32a15b2949977b940d95e7bbe
Reviewed-on: https://chromium-review.googlesource.com/1188318
Commit-Queue: Tonko Sabolčec <tsabolcec@google.com>
Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589898}