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

content/test/test_navigation_observer.cc

Index: content/test/test_navigation_observer.cc
diff --git a/content/test/test_navigation_observer.cc b/content/test/test_navigation_observer.cc
index 89ff36fd575feb8ec98c98463596d8c7442c5a2e..fa572708e6ff1d09534e75d7a79bbb2d3122c51b 100644
--- a/content/test/test_navigation_observer.cc
+++ b/content/test/test_navigation_observer.cc
@@ -84,11 +84,13 @@ void TestNavigationObserver::WaitForObservation(
 }
 
 void TestNavigationObserver::Wait() {
+  MessageLoop::RunID run_id = MessageLoop::current()->GetRunID();
   WaitForObservation(
-      base::Bind(&MessageLoop::Run,
-                 base::Unretained(MessageLoop::current())),
-      base::Bind(&MessageLoop::Quit,
-                 base::Unretained(MessageLoop::current())));
+      base::Bind(&MessageLoop::RunWithID,
+                 base::Unretained(MessageLoop::current()), run_id),
+      base::Bind(&MessageLoop::PostTask,
+                 base::Unretained(MessageLoop::current()),
+                 FROM_HERE, MessageLoop::QuitNowWithIDClosure(run_id)));
 }
 
 TestNavigationObserver::TestNavigationObserver(
@@ -131,8 +133,10 @@ void TestNavigationObserver::Observe(
           ++navigations_completed_ == number_of_navigations_) {
         navigation_started_ = false;
         done_ = true;
-        if (running_)
+        if (running_) {
+          running_ = false;
           done_callback_.Run();
+        }
       }
       break;
     case NOTIFICATION_RENDER_VIEW_HOST_CREATED: