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 11c649c59456d07cad16bcc3106161f1dc0fe89d..bccd36a57c71af72ff9fb9e490fb54efdcd9dbaf 100644
--- a/chrome/test/base/testing_profile.cc
+++ b/chrome/test/base/testing_profile.cc
@@ -377,9 +377,10 @@ void TestingProfile::BlockUntilBookmarkModelLoaded() {
   DCHECK(GetBookmarkModel());
   if (GetBookmarkModel()->IsLoaded())
     return;
-  BookmarkLoadObserver observer;
+  MessageLoop::RunID run_id = MessageLoop::current()->GetRunID();
+  BookmarkLoadObserver observer(run_id);
   GetBookmarkModel()->AddObserver(&observer);
-  MessageLoop::current()->Run();
+  MessageLoop::current()->RunWithID(run_id);
   GetBookmarkModel()->RemoveObserver(&observer);
   DCHECK(GetBookmarkModel()->IsLoaded());
 }