[MD Bookmarks] Refactor window timer mocking.

chrome/test/data/webui/md_bookmarks/md_bookmarks_browsertest.js

Index: chrome/test/data/webui/md_bookmarks/md_bookmarks_browsertest.js
diff --git a/chrome/test/data/webui/md_bookmarks/md_bookmarks_browsertest.js b/chrome/test/data/webui/md_bookmarks/md_bookmarks_browsertest.js
index 524b1be2e9d8b4d87bb7bad21a9b10dbf900928c..89f2f56f62cba5d3b188ab60dade144ec1a57457 100644
--- a/chrome/test/data/webui/md_bookmarks/md_bookmarks_browsertest.js
+++ b/chrome/test/data/webui/md_bookmarks/md_bookmarks_browsertest.js
@@ -28,6 +28,7 @@ MaterialBookmarksBrowserTest.prototype = {
   extraLibraries: PolymerTest.getLibraries(ROOT_PATH).concat([
     'test_command_manager.js',
     'test_store.js',
+    'test_timer_proxy.js',
     'test_util.js',
   ]),
 };