[CUPS] Implement the Webui handler for the printers auto discovery.

chrome/browser/resources/settings/printing_page/cups_add_printer_dialog.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-dropdown-menu/paper-dropdown-menu-light.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-input/paper-input.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-spinner/paper-spinner.html">
 <link rel="import" href="/i18n_setup.html">
 <link rel="import" href="/printing_page/cups_add_printer_dialog_util.html">
 <link rel="import" href="/printing_page/cups_printers_browser_proxy.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="add-printer-discovery-dialog">
   <template>
     <style include="settings-shared">
       add-printer-list {
-        max-height: 350px;
+        max-height: 310px;
         overflow-y: auto;
+        position: absolute;
+        width: 100%;
+      }
+
+      .dialog-body .center {
+        display: flex;
+        justify-content: center;
+        position: absolute;
+        top: 50%;
+        width: 100%;
+      }
+
+      #searchSpinner {
+        position: absolute;
+        top: 80%;
+      }
+
+      #searchSpinner paper-spinner {
+        --paper-spinner-stroke-width: 2px;
+        -webkit-margin-end: 3px;
+        -webkit-margin-start: 20px;
+        height: 15px;
+        width: 15px;
       }
 
       #manuallyAddPrinterButton {
         -webkit-margin-end: 210px;
       }
     </style>
     <add-printer-dialog>
       <div class="dialog-body">
         <add-printer-list printers="[[discoveredPrinters]]"
             selected-printer="{{selectedPrinter}}">
         </add-printer-list>
-        <!-- TODO(xdai): Add the paper-spinner -->
+        <div class="center" id="noPrinterMessage" hidden>
+           $i18n{noPrinterNearbyMessage}
+        </div>
+        <div id="searchSpinner" hidden="[[!discovering_]]">
+          <paper-spinner active="[[discovering_]]"></paper-spinner>
+          <span>$i18n{searchingNearbyPrinters}</span>
+        </div>
       </div>
       <div class="dialog-buttons">
         <paper-button id="manuallyAddPrinterButton"
             on-tap="switchToManualAddDialog_">
           $i18n{manuallyAddPrinterButtonText}
         </paper-button>
         <paper-button class="cancel-button" on-tap="onCancelTap_">
           $i18n{cancelButtonText}
         </paper-button>
         <paper-button class="action-button" id="addPrinterButton"
@@ skipping 307 matching lines @@
     <!-- Manufacturer and Model Dialog -->
     <template is="dom-if" if="[[showManufacturerDialog_]]" restamp>
       <add-printer-manufacturer-model-dialog new-printer="{{newPrinter}}"
           setup-failed="[[setupFailed]]">
       </add-printer-manufacturer-model-dialog>
     </template>
 
   </template>
   <script src="cups_add_printer_dialog.js"></script>
 </dom-module>