List printers managed by extensions in print preview

chrome/browser/resources/print_preview/print_header.js

Index: chrome/browser/resources/print_preview/print_header.js
diff --git a/chrome/browser/resources/print_preview/print_header.js b/chrome/browser/resources/print_preview/print_header.js
index cc0895643fa9fe773bc4d2a7dd8f404017b13e7e..6d623fa7dbed06da3621d9814fbe2d99dcd1bfc8 100644
--- a/chrome/browser/resources/print_preview/print_header.js
+++ b/chrome/browser/resources/print_preview/print_header.js
@@ -142,6 +142,9 @@ cr.define('print_preview', function() {
     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();