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

chrome/test/base/in_process_browser_test.cc

Index: chrome/test/base/in_process_browser_test.cc
diff --git a/chrome/test/base/in_process_browser_test.cc b/chrome/test/base/in_process_browser_test.cc
index b4902133b7ad508e2c93f22bb74721ccd0710985..8e7b6ee3618474159c9e60175f95ed92dfa01aaa 100644
--- a/chrome/test/base/in_process_browser_test.cc
+++ b/chrome/test/base/in_process_browser_test.cc
@@ -373,6 +373,10 @@ void InProcessBrowserTest::RunTestOnMainThreadLoop() {
   autorelease_pool_->Recycle();
 #endif
 
+  // Sometimes tests leave Quit tasks in the MessageLoop (for shame), so let's
+  // run all pending messages here to avoid preempting the QuitBrowsers tasks.
+  ui_test_utils::RunAllPendingInMessageLoop();
+
   QuitBrowsers();
   CHECK(BrowserList::empty());
 }