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ShowSigninCommand is a command to perform a sign in operation.

Modernize other commands to use "instancetype", "NS_DESIGNATED_INITIALIZER",
"NS_UNAVAILABLE" for the initializers and "@synthetize" for the properties.

Remove unused ShowAccountsSettingsCommand.

BUG=429756

Review URL: https://codereview.chromium.org/1124173004

Cr-Commit-Position: refs/heads/master@{#329627}