MD User Manager: Display error message when all profiles are locked.

chrome/browser/resources/md_user_manager/profile_browser_proxy.js

Index: chrome/browser/resources/md_user_manager/profile_browser_proxy.js
diff --git a/chrome/browser/resources/md_user_manager/profile_browser_proxy.js b/chrome/browser/resources/md_user_manager/profile_browser_proxy.js
index 2bc9630db21fd8c1627e2642f895b07115f94be9..55701d67299c5e8ddd9abbc248f62c00911c2e83 100644
--- a/chrome/browser/resources/md_user_manager/profile_browser_proxy.js
+++ b/chrome/browser/resources/md_user_manager/profile_browser_proxy.js
@@ -118,6 +118,15 @@ cr.define('signin', function() {
      */
     switchToProfile: function(profilePath) {
       assertNotReached();
+    },
+
+    /**
+     * Returns a Promise that is resolved if at least one (non-supervised and
+     * non-child) profile is unlocked, and is rejected otherwise.
+     * @return {Promise}
+     */
+    isAtLeastOneProfileUnlocked: function() {

[dpapad, 2016/05/31 19:11:59]

The convention for asynchronous code behavior is the following

Async     -> Sync equivalent
----------------------------
Resolving -> Returning a value.
Rejecting -> Throwing an exception.

Imagine that isAtLeastOneProfileUnlocked() was a synchronous method. Would you
expect it to either return true, or throw an error (instead of returning false)?

Rejecting a Promise should indicate that something went wrong, it should not be
used as a way of communicating a valid result. I suggest changing this method to
always resolve with false/true for the reasons mentioned above.

[Moe, 2016/05/31 20:53:05]

Makes sense. Done.

+      assertNotReached();
     }
   };
 
@@ -183,6 +192,11 @@ cr.define('signin', function() {
     /** @override */
     switchToProfile: function(profilePath) {
       chrome.send('switchToProfile', [profilePath]);
+    },
+
+    /** @override */
+    isAtLeastOneProfileUnlocked: function() {
+      return cr.sendWithPromise('isAtLeastOneProfileUnlocked');
     }
   };