Fix some flaky tests.

content/public/test/browser_test_utils.cc

Index: content/public/test/browser_test_utils.cc
diff --git a/content/public/test/browser_test_utils.cc b/content/public/test/browser_test_utils.cc
index 139301fa6344bb2f3106253aaed144e11cdb2a7e..8de010dd8dec2022596442e61483fedd7da71f74 100644
--- a/content/public/test/browser_test_utils.cc
+++ b/content/public/test/browser_test_utils.cc
@@ -1316,7 +1316,8 @@ bool DOMMessageQueue::WaitForMessage(std::string* message) {
   DCHECK(message);
   if (message_queue_.empty()) {
     // This will be quit when a new message comes in.
-    message_loop_runner_ = new MessageLoopRunner;
+    message_loop_runner_ =
+        new MessageLoopRunner(MessageLoopRunner::QuitMode::IMMEDIATE);
     message_loop_runner_->Run();
   }
   // The queue should not be empty, unless we were quit because of a timeout.