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

chrome/test/base/bookmark_load_observer.h

Index: chrome/test/base/bookmark_load_observer.h
diff --git a/chrome/test/base/bookmark_load_observer.h b/chrome/test/base/bookmark_load_observer.h
index 0619838e4ab160134c4cadfb3526160c5927d460..ee94fb42b40d66366651ee8ddca08496412a1196 100644
--- a/chrome/test/base/bookmark_load_observer.h
+++ b/chrome/test/base/bookmark_load_observer.h
@@ -8,6 +8,7 @@
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
+#include "base/message_loop.h"
 #include "chrome/browser/bookmarks/bookmark_model_observer.h"
 
 // BookmarkLoadObserver is used when blocking until the BookmarkModel
@@ -15,7 +16,7 @@
 // loop is quit.
 class BookmarkLoadObserver : public BookmarkModelObserver {
  public:
-  BookmarkLoadObserver();
+  explicit BookmarkLoadObserver(MessageLoop::RunID run_id);
   virtual ~BookmarkLoadObserver();
 
   virtual void Loaded(BookmarkModel* model, bool ids_reassigned) OVERRIDE;
@@ -40,6 +41,7 @@ class BookmarkLoadObserver : public BookmarkModelObserver {
                                           const BookmarkNode* node) OVERRIDE {}
 
  private:
+  MessageLoop::RunID message_loop_run_id_;
   DISALLOW_COPY_AND_ASSIGN(BookmarkLoadObserver);
 };