Add onGetUsbPrinterInfoRequested event to printerProvider API.

chrome/browser/resources/print_preview/data/destination_store.js

Index: chrome/browser/resources/print_preview/data/destination_store.js
diff --git a/chrome/browser/resources/print_preview/data/destination_store.js b/chrome/browser/resources/print_preview/data/destination_store.js
index b261ce27879e1453a46faef2725010512466a542..6f52f234d593b34e24ddd3a133d9404ac0ee2635 100644
--- a/chrome/browser/resources/print_preview/data/destination_store.js
+++ b/chrome/browser/resources/print_preview/data/destination_store.js
@@ -1008,7 +1008,12 @@ cr.define('print_preview', function() {
      * @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);
       }));