Add support for starting an extension print job to print preview

chrome/browser/resources/print_preview/print_header.js

 // Copyright (c) 2012 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.
 
 cr.define('print_preview', function() {
   'use strict';
 
   /**
    * Creates a PrintHeader object. This object encapsulates all the elements
    * and logic related to the top part of the left pane in print_preview.html.
@@ skipping 124 matching lines @@
           this.onTicketChange_.bind(this));
     },
 
     /**
      * Updates Print Button state.
      * @private
      */
     updatePrintButtonEnabledState_: function() {
       this.getChildElement('button.print').disabled =
           this.destinationStore_.selectedDestination == null ||
-          // TODO(tbarzic): Remove this when print request for extension
-          // destinations is wired up.
-          this.destinationStore_.selectedDestination.isExtension ||
           !this.isEnabled_ ||
           !this.isPrintButtonEnabled_ ||
           !this.printTicketStore_.isTicketValid();
     },
 
     /**
      * Updates the summary element based on the currently selected user options.
      * @private
      */
     updateSummary_: function() {
@@ skipping 111 matching lines @@
         this.getChildElement('button.print').focus();
       }
     }
   };
 
   // Export
   return {
     PrintHeader: PrintHeader
   };
 });