ChromeOS OOBE: Implement network selection screen "Back" button.

chrome/browser/resources/chromeos/login/oobe_welcome.html

 <!-- Copyright 2016 The Chromium Authors. All rights reserved.
      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file. -->
 
 <link rel="import" href="chrome://resources/cr_elements/network/cr_network_icon.html">
 <link rel="import" href="chrome://resources/cr_elements/network/cr_network_select.html">
 <link rel="import" href="chrome://resources/cr_elements/network/cr_onc_types.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-iconset-svg/iron-iconset-svg.html">
 
 <iron-iconset-svg name="oobe-welcome" size="24">
   <svg>
     <defs>
-      <g id="add"><path d="M19 13h-6v6h-2v-6H5v-2h6V5h2v6h6v2z"></path></g>
       <g id="wifi"><path d="M1 9l2 2c4.97-4.97 13.03-4.97 18 0l2-2C16.93 2.93 7.08 2.93 1 9zm8 8l3 3 3-3c-1.65-1.66-4.34-1.66-6 0zm-4-4l2 2c2.76-2.76 7.24-2.76 10 0l2-2C15.14 9.14 8.87 9.14 5 13z"></path></g>
     </defs>
   </svg>
 </iron-iconset-svg>
 
+<iron-iconset-svg name="oobe-welcome-20" size="20">
+  <svg>
+    <defs>
+      <g id="add-cellular" fill="none" fill-rule="evenodd">
+        <path d="M1 0h18v18H1"></path>
+        <path d="M9.207 16H2L17 1v8.6c-.75-.384-1.6-.6-2.5-.6C11.462 9 9 11.462 9 14.5c0 .52.072 1.023.207 1.5z" fill-opacity=".3" fill="#000"></path>
+        <path d="M16 14v-3h-2v3h-3v2h3v3h2v-3h3v-2h-3z" fill="#000"></path>
+      </g>
+      <g id="add-wifi" fill="none" fill-rule="evenodd">
+        <g fill="#000">
+          <path d="M10.044 17.25L1.5 6.26c.33-.255 3.593-3.01 8.5-3.01 4.9 0 8.17 2.755 8.5 3.01L16.81 8.46c-.49-.156-1.012-.24-1.552-.24-2.904 0-5.258 2.426-5.258 5.42 0 .997.262 1.932.718 2.735l-.674.877z" fill-opacity=".3"></path>
+          <path d="M17 13v-3h-2v3h-3v2h3v3h2v-3h3v-2h-3z"></path>
+        </g>
+      </g>
+      <g id="add-proxy" fill="none" fill-rule="evenodd">
+        <g fill="#000">
+          <path d="M16 14v-3h-2v3h-3v2h3v3h2v-3h3v-2h-3z"></path>
+          <path d="M8.04 15.48c-.74-.1-1.43-.33-2.06-.66-.04-.02-.07-.03-.11-.05l-.24-.15c-.09-.05-.18-.11-.27-.17-.06-.04-.11-.08-.17-.13-.11-.07-.21-.15-.31-.23-.05-.04-.09-.08-.14-.12-.1-.1-.2-.19-.3-.29l-.12-.12c-.09-.11-.19-.21-.28-.33-.04-.04-.07-.08-.1-.12-.09-.12-.18-.24-.26-.36-.03-.04-.06-.09-.09-.13-.08-.13-.15-.25-.22-.38-.03-.05-.06-.1-.08-.15-.07-.13-.13-.26-.19-.4-.02-.05-.05-.1-.07-.16-.05-.13-.1-.27-.14-.41-.02-.05-.04-.11-.06-.17-.04-.14-.07-.28-.1-.42-.02-.06-.04-.13-.05-.19-.03-.15-.05-.29-.06-.44l-.03-.18c-.02-.21-.03-.42-.03-.64 0-.5.06-.97.16-1.44l3.75 3.86v.81c0 .88.71 1.61 1.57 1.61v1.56zm3.13-5.4v-.19c0-.45-.35-.81-.78-.81h-4.7V7.47h1.57c.43 0 .78-.36.78-.81V5.05H9.6c.86 0 1.57-.73 1.57-1.61v-.33c2.06.86 3.57 2.82 3.86 5.16h-.01c0 .04.01.07.01.1h.12c.51 0 1 .07 1.47.21 0-.1 0-.21-.01-.31-.32-3.24-2.51-5.93-5.44-6.88h-.01c-.03-.01-.06-.02-.08-.03-.2-.06-.4-.11-.6-.16-.07-.01-.14-.02-.2-.04-.18-.03-.36-.06-.53-.08-.07-.01-.14-.02-.2-.02-.24-.03-.48-.04-.73-.04C4.5 1.02 1 4.63 1 9.08c0 4.45 3.5 8.06 7.82 8.06.51 0 1-.05 1.48-.14-.47-.85-.74-1.83-.74-2.87 0-1.58.61-3.01 1.61-4.05z" fill-opacity=".3"></path>
+        </g>
+      </g>
+    </defs>
+  </svg>
+</iron-iconset-svg>
+
 <dom-module name="oobe-welcome-md">
   <template>
     <link rel="stylesheet" href="oobe_dialog_host.css">
     <link rel="stylesheet" href="oobe_welcome.css">
     <link rel="stylesheet" href="oobe_dialog_parameters.css">
     <oobe-welcome-dialog id="welcomeScreen" hidden="[[!welcomeScreenShown]]"
         current-language="[[currentLanguage]]"
         on-language-button-clicked="onWelcomeSelectLanguageButtonClicked_"
         on-accessibility-button-clicked="onWelcomeAccessibilityButtonClicked_"
         on-next-button-clicked="onWelcomeNextButtonClicked_">
@@ skipping 87 matching lines @@
           <span class="unchecked-value" i18n-content="virtualKeyboardOptionOff">
           </span>
         </oobe-a11y-option>
       </div>
       <div class="bottom-buttons layout horizontal end-justified">
         <oobe-text-button inverse on-tap="closeAccessibilitySection_">
           <div i18n-content="oobeOKButtonText"></div>
         </oobe-text-button>
       </div>
     </oobe-dialog>
-    <oobe-dialog hidden="[[!networkSelectionScreenShown]]">
+    <oobe-dialog id="networkSelectionScreen"
+        hidden="[[!networkSelectionScreenShown]]" has-buttons>
       <iron-icon icon="oobe-welcome:wifi" class="oobe-icon"></iron-icon>
       <div class="header">
         <h1 class="title" i18n-content="networkSectionTitle"></h1>
         <div class="subtitle" i18n-content="networkSectionHint"></div>
       </div>
       <div class="footer layout vertical">
         <cr-network-select id="networkSelect"
             on-network-connected="onNetworkConnected_"
             on-network-item-selected="onNetworkListNetworkItemSelected_"
             on-custom-item-selected="onNetworkListCustomItemSelected_"
             custom-items="[[_getNetworkCustomItems()]]"
-            max-height="385">
+            max-height="307">
         </cr-network-select>
       </div>
+      <div class="bottom-buttons layout horizontal start-justified">
+        <oobe-back-button on-tap="onNetworkSelectionBackButtonPressed_">
+        </oobe-back-button>
+      </div>
     </oobe-dialog>
   </template>
 </dom-module>