MD Settings: implement support for extension-controlled startup URLs

chrome/browser/resources/settings/on_startup_page/on_startup_page.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="/controls/controlled_radio_button.html">
 <link rel="import" href="/controls/extension_controlled_indicator.html">
 <link rel="import" href="/controls/settings_radio_group.html">
 <link rel="import" href="/on_startup_page/on_startup_browser_proxy.html">
 <link rel="import" href="/on_startup_page/startup_urls_page.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-on-startup-page">
@@ skipping 20 matching lines @@
           $i18n{onStartupContinue}
         </controlled-radio-button>
         <controlled-radio-button name="[[prefValues_.OPEN_SPECIFIC]]"
             pref="[[prefs.session.restore_on_startup]]">
           $i18n{onStartupOpenSpecific}
         </controlled-radio-button>
       </settings-radio-group>
     </div>
     <template is="dom-if"
         if="[[showStartupUrls_(prefs.session.restore_on_startup.value)]]">
-      <settings-startup-urls-page></settings-startup-urls-page>
+      <settings-startup-urls-page prefs="[[prefs]]">

[dpapad, 2016/11/18 23:46:28]

In almost all cases where we pass |prefs| we use two-mway binding "{{prefs}}".
Is this different? Or should we use 1-way everywhere else? 

[Dan Beam, 2016/11/18 23:51:04]

i don't set prefs.* in the startup pages.  that's the only reason notify: true
is required

+      </settings-startup-urls-page>
     </template>
   </template>
   <script src="on_startup_page.js"></script>
 </dom-module>