[Smart Lock] Add a setting to require close proximity in order for unlocking to work.

chrome/browser/resources/options/browser_options.html

 <div id="settings" class="page" hidden>
   <header>
     <h1 i18n-content="settingsTitle"></h1>
   </header>
   <include src="reset_profile_settings_banner.html">
   <include src="automatic_settings_reset_banner.html">
 <if expr="chromeos">
   <include src="secondary_user_banner.html">
   <section>
     <div id="network-section-header" class="section-header">
@@ skipping 528 matching lines @@
             i18n-values="href:easyUnlockLearnMoreURL"></a>
       </div>
       <button id="easy-unlock-setup-button"
           i18n-content="easyUnlockSetupButton"></button>
     </div>
     <!-- Options shown when the user has set up Easy Unlock -->
     <div id='easy-unlock-enable' hidden>
       <div class="settings-row">
         <span i18n-content="easyUnlockDescription"></span>
         <a target="_blank" i18n-content="learnMore"
-            i18n-values="href:easyUnlockLearnMoreURL"></a>
+           i18n-values="href:easyUnlockLearnMoreURL"></a>
+        <div class="checkbox">
+          <label>
+            <input type="checkbox"
+                   metric="Options_EasyUnlockRequireProximity"
+                   pref="easy_unlock.proximity_required">
+            <span i18n-content="easyUnlockRequireProximityLabel"></span>
+          </label>
+        </div>
       </div>
       <button id="easy-unlock-turn-off-button"
           i18n-content="easyUnlockTurnOffButton"></button>
     </div>
   </section>
   <section id="web-content-section">
     <h3 i18n-content="advancedSectionTitleContent"></h3>
     <div>
       <div id="website-settings-section" class="settings-row" hidden>
         <button id="website-management-button"
@@ skipping 393 matching lines @@
       </button>
     </div>
   </section>
   </div>  <!-- advanced-settings-container -->
   </div>  <!-- advanced-settings -->
   <footer>
     <a is="action-link" id="advanced-settings-expander"
         i18n-content="showAdvancedSettings"></a>
   </footer>
 </div>