[EasyUnlock] Add option in chrome://settings to enable EasyUnlock for login.

Screenshot:
https://screenshot.googleplex.com/BV3uweR38fL.png

See mocks at:
https://docs.google.com/presentation/d/1GnKn5ca_KrM_BTQB-He9cCmzAQtwVF6JJJ6Lo6i4gqI/edit#slide=id.g4af27b4ff_35243

BUG=748288

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I6f8f0b92164d546c1670816a76d3289d648955f1
Reviewed-on: https://chromium-review.googlesource.com/582342Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Gustavo Sacomoto <sacomoto@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491185}

chrome/browser/extensions/api/settings_private/prefs_util.cc

@@ -229,6 +229,8 @@ const PrefsUtil::TypedPrefMap& PrefsUtil::GetWhitelistedKeys() {
       settings_private::PrefType::PREF_TYPE_BOOLEAN;
   (*s_whitelist)[proximity_auth::prefs::kEasyUnlockProximityThreshold] =
       settings_private::PrefType::PREF_TYPE_NUMBER;
+  (*s_whitelist)[proximity_auth::prefs::kProximityAuthIsChromeOSLoginEnabled] =
+      settings_private::PrefType::PREF_TYPE_BOOLEAN;
 
   // Accessibility.
   (*s_whitelist)[::prefs::kAccessibilitySpokenFeedbackEnabled] =

chrome/browser/resources/settings/people_page/lock_screen.html

@@ -49,9 +49,17 @@
         margin: auto;
       }
 
-      .easy-unlock-nested-settings {
+      #easyUnlockSettingsCollapsable {
         @apply(--settings-list-frame-padding);
       }
+
+      .no-padding {
+        padding: 0;
+      }
+
+      .underbar {
+        border-bottom: var(--settings-separator-line);
+      }
     </style>
 
     <div>
@@ -155,8 +163,16 @@
             </paper-button>
           </template>
         </div>
-        <iron-collapse opened="[[easyUnlockEnabled_]]">
-          <div class="settings-box continuation easy-unlock-nested-settings">
+        <iron-collapse opened="[[easyUnlockEnabled_]]"
+            id="easyUnlockSettingsCollapsable">
+          <div class="settings-box continuation no-padding underbar">
+            <settings-toggle-button
+              class="start"
+              pref="{{prefs.proximity_auth.is_chromeos_login_enabled}}"
+              label="$i18n{easyUnlockAllowSignInLabel}">
+            </settings-toggle-button>
+          </div>
+          <div class="settings-box continuation no-padding">
             <div class="start">
               $i18n{easyUnlockProximityThresholdLabel}
             </div>

chrome/browser/ui/webui/settings/md_settings_localized_strings_provider.cc

@@ -903,6 +903,8 @@ void AddEasyUnlockStrings(content::WebUIDataSource* html_source) {
       {"easyUnlockTurnOffErrorMessage",
        IDS_SETTINGS_EASY_UNLOCK_TURN_OFF_ERROR_MESSAGE},
       {"easyUnlockTurnOffRetryButton", IDS_SETTINGS_EASY_UNLOCK_TURN_OFF_RETRY},
+      {"easyUnlockAllowSignInLabel",
+       IDS_SETTINGS_EASY_UNLOCK_ALLOW_SIGN_IN_LABEL},
       {"easyUnlockProximityThresholdLabel",
        IDS_SETTINGS_EASY_UNLOCK_PROXIMITY_THRESHOLD_LABEL},
       {"easyUnlockProximityThresholdVeryClose",