MD Settings: add 'preserve-focus' flag to fix iron-list focus bugs.

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

 <link rel="import" href="chrome://resources/html/polymer.html">
 
 <link rel="import" href="chrome://resources/html/action_link.html">
 <link rel="import" href="chrome://resources/html/action_link_css.html">
 <link rel="import" href="chrome://resources/html/assert.html">
 <link rel="import" href="chrome://resources/html/cr/ui/focus_without_ink.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/iron-flex-layout-classes.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-list/iron-list.html">
 <link rel="import" href="chrome://resources/html/web_ui_listener_behavior.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_scrollable_behavior.html">
@@ skipping 14 matching lines @@
       #container iron-list > settings-startup-url-entry:not(:first-of-type) {
         border-top: var(--settings-separator-line);
       }
 
       #container settings-startup-url-entry {
         cursor: default;
       }
     </style>
     <div id="outer" class="layout vertical flex vertical-list">
       <div id="container" class="scroll-container" scrollable>
-        <iron-list items="[[startupPages_]]" scroll-target="container">
+        <iron-list items="[[startupPages_]]" scroll-target="container"
+            preserve-focus>
           <template>
             <settings-startup-url-entry model="[[item]]"
                 tabindex$="[[tabIndex]]" iron-list-tab-index="[[tabIndex]]"
                 last-focused="{{lastFocused_}}"
                 editable="[[!prefs.session.startup_urls.controlledBy]]">
             </settings-startup-url-entry>
           </template>
         </iron-list>
       </div>
       <template is="dom-if" if="[[!prefs.session.startup_urls.controlledBy]]"
@@ skipping 21 matching lines @@
       </template>
     </div>
     <template is="dom-if" if="[[showStartupUrlDialog_]]" restamp>
       <settings-startup-url-dialog model="[[startupUrlDialogModel_]]"
           on-close="destroyUrlDialog_">
       </settings-startup-url-dialog>
     </template>
   </template>
   <script src="startup_urls_page.js"></script>
 </dom-module>