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

chrome/test/base/test_web_dialog_observer.cc

Index: chrome/test/base/test_web_dialog_observer.cc
diff --git a/chrome/test/base/test_web_dialog_observer.cc b/chrome/test/base/test_web_dialog_observer.cc
index 20565920fc7ec21f941d5be96b38f361dfbf48b4..10cc67b0741b29d8e7639aff1b7f35268d31db0d 100644
--- a/chrome/test/base/test_web_dialog_observer.cc
+++ b/chrome/test/base/test_web_dialog_observer.cc
@@ -42,7 +42,7 @@ void TestWebDialogObserver::Observe(
       // If the message loop is running stop it.
       if (running_) {
         running_ = false;
-        MessageLoopForUI::current()->Quit();
+        MessageLoopForUI::current()->QuitAfterPending();
       }
       break;
     default: