Add onGetUsbPrinterInfoRequested event to printerProvider API.

chrome/browser/resources/print_preview/data/destination_store.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';
 
   /**
    * A data store that stores destinations and dispatches events when the data
    * store changes.
@@ skipping 990 matching lines @@
     /**
      * Called when an extension responds to a getExtensionDestinations
      * request.
      * @param {Object} event Contains information about list of printers
      *     reported by the extension.
      *     {@code done} parameter is set iff this is the final list of printers
      *     returned as part of getExtensionDestinations request.
      * @private
      */
     onExtensionPrintersAdded_: function(event) {
-      this.insertDestinations_(event.printers.map(function(printer) {
+      // Filter out printers enumerated from providers' "usbPrinters" manifest
+      // key for now.
+      var filteredPrinters = event.printers.filter(function(printer) {

[Vitaly Buka (NO REVIEWS), 2015/05/21 00:54:43]

this piece for another CL?

[Reilly Grant (use Gerrit), 2015/05/21 00:59:55]

This piece keeps the changes in this CL from having a visible effect until the
WebUI is ready to handle the new entries. The USB printer list is inserted by
the printProvider API layer, not the preview integration layer.

+        return 'extensionId' in printer;
+      });
+      this.insertDestinations_(filteredPrinters.map(function(printer) {
         return print_preview.ExtensionDestinationParser.parse(printer);
       }));
 
       if (event.done && this.isExtensionDestinationSearchInProgress_) {
         clearTimeout(this.extensionSearchTimeout_);
         this.endExtensionPrinterSearch_();
       }
     },
 
     /**
@@ skipping 71 matching lines @@
       return id == this.appState_.selectedDestinationId &&
              origin == this.appState_.selectedDestinationOrigin;
     }
   };
 
   // Export
   return {
     DestinationStore: DestinationStore
   };
 });