mojo api for view based lockscreen

ash/public/interfaces/lock_screen.mojom

Index: ash/public/interfaces/lock_screen.mojom
diff --git a/ash/public/interfaces/lock_screen.mojom b/ash/public/interfaces/lock_screen.mojom
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index 0000000000000000000000000000000000000000..e563f883c61ee05e037b19885da31d403906f817
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+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module ash.mojom;
+
+import "components/signin/public/interfaces/account_id.mojom";
+
+// Allows clients (e.g. Chrome browser) to control the ash lock screen.
+interface LockScreen {
+  // Sets the client interface.
+  SetClient(LockScreenClient client);
+
+  // Shows error message in the ash lock screen.
+  ShowErrorMessage();

[xiyuan, 2017/05/11 21:17:33]

Do we need the error message to be displayed?

[xiaoyinh(OOO Sep 11-29), 2017/05/13 00:29:49]

chrome will request to show error message when the unlock authentication is
failed. I think this will be needed as long as we are  authenticating in chrome.

[xiyuan, 2017/05/15 20:39:41]

Sorry, I should be more clear. I meant how would LockScreen know what error
message to be displayed?

+};
+
+// Allows ash lock screen to control a client (e.g. Chrome browser). Requests
+// often involve preferences or talk to cryptohome that is not available to ash.
+interface LockScreenClient {
+  // Request user authentication in chrome.

[xiyuan, 2017/05/11 21:17:33]

nit: Document what happens when auth succeeds or fails.

[xiaoyinh(OOO Sep 11-29), 2017/05/13 00:29:49]

Done.

+  AuthenticateUser(signin.mojom.AccountId account_id,
+                   string password,

[xiyuan, 2017/05/11 21:17:33]

Transfer plain password over mojo makes me nervous. We probably should do the
key hashing before sending it back.

[xiaoyinh(OOO Sep 11-29), 2017/05/13 00:29:49]

Thanks. The new patch hash the password in LockScreenController before sending
it to chrome. In order to get the system salt, I add +chromeos/cryptohome to
ash/DEPS. Please let me know if this is a bad idea.

+		   bool authenticated_by_pin);
+};