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

chrome/test/base/bookmark_load_observer.cc

Index: chrome/test/base/bookmark_load_observer.cc
diff --git a/chrome/test/base/bookmark_load_observer.cc b/chrome/test/base/bookmark_load_observer.cc
index 5b80070149ea290b3e8710262d449a00f40b20ec..2b405009595dde356eacaad367817f0d00c1ad7e 100644
--- a/chrome/test/base/bookmark_load_observer.cc
+++ b/chrome/test/base/bookmark_load_observer.cc
@@ -4,12 +4,13 @@
 
 #include "chrome/test/base/bookmark_load_observer.h"
 
-#include "base/message_loop.h"
+#include "chrome/test/base/ui_test_utils.h"
 
-BookmarkLoadObserver::BookmarkLoadObserver() {}
+BookmarkLoadObserver::BookmarkLoadObserver(MessageLoop::RunID run_id)
+    : message_loop_run_id_(run_id) {}
 
 BookmarkLoadObserver::~BookmarkLoadObserver() {}
 
 void BookmarkLoadObserver::Loaded(BookmarkModel* model, bool ids_reassigned) {
-  MessageLoop::current()->Quit();
+  ui_test_utils::QuitMessageLoopWithID(message_loop_run_id_);
 }