Always use an async TaskScheduler in TestBrowserThreadBundle.

content/public/test/test_browser_thread_bundle.h

Index: content/public/test/test_browser_thread_bundle.h
diff --git a/content/public/test/test_browser_thread_bundle.h b/content/public/test/test_browser_thread_bundle.h
index 970c9404ef95853db8495f154fd002210b5a0865..15cdba8f5c6b350876bfb172fe04a15de5dac492 100644
--- a/content/public/test/test_browser_thread_bundle.h
+++ b/content/public/test/test_browser_thread_bundle.h
@@ -9,26 +9,43 @@
 // the first member variable in test classes, so it is destroyed last, and the
 // test threads always exist from the perspective of other classes.
 //
-// By default, all of the created TestBrowserThreads and the task scheduler will
-// be backed by a single shared MessageLoop. If a test truly needs separate
-// threads, it can do so by passing the appropriate combination of option values
-// during the TestBrowserThreadBundle construction.
+// By default, all of the created TestBrowserThreads will be backed by a single
+// shared MessageLoop. If a test truly needs separate threads, it can do so by
+// passing the appropriate combination of option values during the
+// TestBrowserThreadBundle construction. TaskScheduler and blocking pool tasks
+// always run on dedicated threads.
 //
-// To synchronously run tasks posted to task scheduler or to TestBrowserThreads
-// that use the shared MessageLoop, call RunLoop::Run/RunUntilIdle() on the
-// thread where the TestBrowserThreadBundle lives. The destructor of
-// TestBrowserThreadBundle runs remaining TestBrowserThreads tasks, remaining
-// blocking pool tasks, and remaining BLOCK_SHUTDOWN task scheduler tasks.
+// To synchronously run tasks from the shared MessageLoop:
+//
+// ... until there are no undelayed tasks in the shared MessageLoop:
+//    base::RunLoop::RunUntilIdle();
+//
+// ... until there are no undelayed tasks in the shared MessageLoop, in
+// TaskScheduler or in the blocking pool (excluding tasks not posted from the
+// shared MessageLoop's thread, TaskScheduler or the blocking pool):
+//    content::RunAllBlockingPoolTasksUntilIdle();
+//
+// ... until a condition is met:
+//    base::RunLoop run_loop;
+//    // Runs until a task running in the shared MessageLoop calls
+//    // run_loop.Quit() or runs run_loop.QuitClosure() (&run_loop or
+//    // run_loop.QuitClosure() must be kept somewhere accessible by that task).
+//    run_loop.Run();
+//
+// To wait until there are no pending undelayed tasks in TaskScheduler or in the
+// blocking pool, without running tasks from the shared MessageLoop:
+//    base::TaskScheduler::GetInstance()->FlushForTesting();
+//    // Note: content::BrowserThread::GetBlockingPool()->FlushForTesting() is
+//    // equivalent but deprecated.
+//
+// The destructor of TestBrowserThreadBundle runs remaining TestBrowserThreads
+// tasks, remaining blocking pool tasks, and remaining BLOCK_SHUTDOWN task
+// scheduler tasks.
 //
 // If a test needs a MessageLoopForIO on the main thread, it should use the
-// IO_MAINLOOP option. This also allows task scheduler tasks to use
-// FileDescriptorWatcher. Most of the time, IO_MAINLOOP avoids needing to use a
+// IO_MAINLOOP option. Most of the time, IO_MAINLOOP avoids needing to use a
 // REAL_IO_THREAD.
 //
-// If a test needs a TaskScheduler that runs tasks on a dedicated thread, it
-// should use REAL_TASK_SCHEDULER. Usage of this option should be justified as
-// it is easier to understand and debug a single-threaded unit test.
-//
 // For some tests it is important to emulate real browser startup. During real
 // browser startup, the main MessageLoop is created before other threads.
 // Passing DONT_CREATE_THREADS to constructor will delay creating other threads
@@ -48,7 +65,6 @@ namespace base {
 class MessageLoop;
 namespace test {
 class ScopedAsyncTaskScheduler;
-class ScopedTaskScheduler;
 }  // namespace test
 }  // namespace base
 
@@ -67,8 +83,7 @@ class TestBrowserThreadBundle {
     REAL_DB_THREAD = 1 << 1,
     REAL_FILE_THREAD = 1 << 2,
     REAL_IO_THREAD = 1 << 3,
-    REAL_TASK_SCHEDULER = 1 << 4,
-    DONT_CREATE_THREADS = 1 << 5,
+    DONT_CREATE_THREADS = 1 << 4,
   };
 
   TestBrowserThreadBundle();
@@ -86,7 +101,6 @@ class TestBrowserThreadBundle {
   std::unique_ptr<base::MessageLoop> message_loop_;
   std::unique_ptr<base::test::ScopedAsyncTaskScheduler>
       scoped_async_task_scheduler_;
-  std::unique_ptr<base::test::ScopedTaskScheduler> scoped_task_scheduler_;
   std::unique_ptr<TestBrowserThread> ui_thread_;
   std::unique_ptr<TestBrowserThread> db_thread_;
   std::unique_ptr<TestBrowserThread> file_thread_;