Add support for starting an extension print job to print preview

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

Index: chrome/browser/resources/print_preview/data/print_ticket_store.js
diff --git a/chrome/browser/resources/print_preview/data/print_ticket_store.js b/chrome/browser/resources/print_preview/data/print_ticket_store.js
index 0ca75a4ac5a076a62758b4ac7ec27e00c4344da1..e2e69417a3529e3bd72354623f722972327104e7 100644
--- a/chrome/browser/resources/print_preview/data/print_ticket_store.js
+++ b/chrome/browser/resources/print_preview/data/print_ticket_store.js
@@ -406,9 +406,10 @@ cr.define('print_preview', function() {
      * @return {!Object} Google Cloud Print print ticket.
      */
     createPrintTicket: function(destination) {
-      assert(!destination.isLocal || destination.isPrivet,
+      assert(!destination.isLocal ||
+             destination.isPrivet || destination.isExtension,
              'Trying to create a Google Cloud Print print ticket for a local ' +
-                 ' non-privet destination');
+                 ' non-privet and non-extension destination');
 
       assert(destination.capabilities,
              'Trying to create a Google Cloud Print print ticket for a ' +