Add plugin stub for print preview tests

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

Index: chrome/test/data/webui/print_preview/print_preview_ui_browsertest.js
diff --git a/chrome/test/data/webui/print_preview/print_preview_ui_browsertest.js b/chrome/test/data/webui/print_preview/print_preview_ui_browsertest.js
index 929f107e09c5c5e0a795dd5708c4def9f516a64a..af6eb5cbd7f6fdb1ce6cb1c31b2dae939fdd3a74 100644
--- a/chrome/test/data/webui/print_preview/print_preview_ui_browsertest.js
+++ b/chrome/test/data/webui/print_preview/print_preview_ui_browsertest.js
@@ -58,6 +58,7 @@ PrintPreviewUIBrowserTest.prototype = {
     ROOT_PATH + 'chrome/test/data/webui/test_browser_proxy.js',
     'print_preview_tests.js',
     'native_layer_stub.js',
+    'plugin_stub.js',
   ],
 };