Add base::RunLoop and update ui_test_utils to use it to reduce flakiness

chrome/test/base/testing_profile.cc

Index: chrome/test/base/testing_profile.cc
diff --git a/chrome/test/base/testing_profile.cc b/chrome/test/base/testing_profile.cc
index a652086e73846155e8ee5861ac8695e6a30af1c4..8f303428cc6e8a48a9e4eaea8811194050377591 100644
--- a/chrome/test/base/testing_profile.cc
+++ b/chrome/test/base/testing_profile.cc
@@ -378,9 +378,10 @@ void TestingProfile::BlockUntilBookmarkModelLoaded() {
   DCHECK(GetBookmarkModel());
   if (GetBookmarkModel()->IsLoaded())
     return;
-  BookmarkLoadObserver observer;
+  MessageLoop::RunLoop run_loop;
+  BookmarkLoadObserver observer(run_loop.AsWeakPtr());
   GetBookmarkModel()->AddObserver(&observer);
-  MessageLoop::current()->Run();
+  run_loop.Run();
   GetBookmarkModel()->RemoveObserver(&observer);
   DCHECK(GetBookmarkModel()->IsLoaded());
 }