Add plugin stub for print preview tests

chrome/test/data/webui/print_preview/print_preview_tests.js

Index: chrome/test/data/webui/print_preview/print_preview_tests.js
diff --git a/chrome/test/data/webui/print_preview/print_preview_tests.js b/chrome/test/data/webui/print_preview/print_preview_tests.js
index e874df6fd7e62fdf2e848e68b48262973ed18f50..b38e99f64b2a655caa09b2328e8af6fd0046935b 100644
--- a/chrome/test/data/webui/print_preview/print_preview_tests.js
+++ b/chrome/test/data/webui/print_preview/print_preview_tests.js
@@ -302,7 +302,9 @@ cr.define('print_preview_test', function() {
       print_preview.NativeLayer.setInstance(nativeLayer);
       printPreview = new print_preview.PrintPreview();
       previewArea = printPreview.getPreviewArea();
-      previewArea.setIsBrowserTest(true);
+      previewArea.plugin_ = new print_preview.PDFPluginStub(previewArea);
+      previewArea.plugin_.setLoadCallback(
+          previewArea.onPluginLoad_.bind(previewArea));
     });
 
     // Test some basic assumptions about the print preview WebUI.