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

chrome/browser/referrer_policy_browsertest.cc

Index: chrome/browser/referrer_policy_browsertest.cc
diff --git a/chrome/browser/referrer_policy_browsertest.cc b/chrome/browser/referrer_policy_browsertest.cc
index df69f972d536b4ff369fb92e72fb1066e4dbfd2d..a25b5aecba16357a77a0f2d259af566c094252d6 100644
--- a/chrome/browser/referrer_policy_browsertest.cc
+++ b/chrome/browser/referrer_policy_browsertest.cc
@@ -254,8 +254,7 @@ IN_PROC_BROWSER_TEST_F(ReferrerPolicyTest, HttpsMiddleClickTargetBlankOrigin) {
 // Context menu, from HTTP to HTTP.
 IN_PROC_BROWSER_TEST_F(ReferrerPolicyTest, MAYBE_ContextMenuOrigin) {
   ContextMenuNotificationObserver context_menu_observer(
-      IDC_CONTENT_CONTEXT_OPENLINKNEWTAB,
-      chrome::NOTIFICATION_TAB_ADDED);
+      IDC_CONTENT_CONTEXT_OPENLINKNEWTAB);
   RunReferrerTest("origin", false, false, false, true,
                   WebKit::WebMouseEvent::ButtonRight,
                   EXPECT_ORIGIN_AS_REFERRER);
@@ -264,8 +263,7 @@ IN_PROC_BROWSER_TEST_F(ReferrerPolicyTest, MAYBE_ContextMenuOrigin) {
 // Context menu, from HTTPS to HTTP.
 IN_PROC_BROWSER_TEST_F(ReferrerPolicyTest, MAYBE_HttpsContextMenuOrigin) {
   ContextMenuNotificationObserver context_menu_observer(
-      IDC_CONTENT_CONTEXT_OPENLINKNEWTAB,
-      chrome::NOTIFICATION_TAB_ADDED);
+      IDC_CONTENT_CONTEXT_OPENLINKNEWTAB);
   RunReferrerTest("origin", true, false, false, true,
                   WebKit::WebMouseEvent::ButtonRight,
                   EXPECT_ORIGIN_AS_REFERRER);
@@ -351,8 +349,7 @@ IN_PROC_BROWSER_TEST_F(ReferrerPolicyTest,
 // Context menu, from HTTP to HTTP via server redirect.
 IN_PROC_BROWSER_TEST_F(ReferrerPolicyTest, MAYBE_ContextMenuRedirect) {
   ContextMenuNotificationObserver context_menu_observer(
-      IDC_CONTENT_CONTEXT_OPENLINKNEWTAB,
-      chrome::NOTIFICATION_TAB_ADDED);
+      IDC_CONTENT_CONTEXT_OPENLINKNEWTAB);
   RunReferrerTest("origin", false, false, true, true,
                   WebKit::WebMouseEvent::ButtonRight,
                   EXPECT_ORIGIN_AS_REFERRER);
@@ -361,8 +358,7 @@ IN_PROC_BROWSER_TEST_F(ReferrerPolicyTest, MAYBE_ContextMenuRedirect) {
 // Context menu, from HTTPS to HTTP via server redirect.
 IN_PROC_BROWSER_TEST_F(ReferrerPolicyTest, MAYBE_HttpsContextMenuRedirect) {
   ContextMenuNotificationObserver context_menu_observer(
-      IDC_CONTENT_CONTEXT_OPENLINKNEWTAB,
-      chrome::NOTIFICATION_TAB_ADDED);
+      IDC_CONTENT_CONTEXT_OPENLINKNEWTAB);
   RunReferrerTest("origin", true, false, true, true,
                   WebKit::WebMouseEvent::ButtonRight,
                   EXPECT_ORIGIN_AS_REFERRER);