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..1476e233dda4ed0d35d270cbdcd440a3ef9ef857 100644
--- a/content/public/test/test_browser_thread_bundle.h
+++ b/content/public/test/test_browser_thread_bundle.h
@@ -9,26 +9,40 @@
 // 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:

[robliao, 2017/04/03 17:53:05]

Should there be this many ways to flush tasks or will this be simplified later
(do tests today take advantage of these distinctions)?

[gab, 2017/04/03 18:14:14]

I think this distinction matters (and I'm making it in ScopedTaskEnvironment).
Ultimately there should "run main thread" and "run all" I think. Anything in
between (i.e. run a specific sequence) ought to be done by manually posting
WaitableEvents IMO.

[robliao, 2017/04/03 18:17:27]

Presented that way, I might be able to buy it. We can have this discussion on a
separate thread.

[fdoray, 2017/04/03 19:18:32]

I expect most that most of the comments added in this CL will become obsolete
when ScopedTaskEnvironment is introduced.

+//    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;
+//    RegisterCallbackToRunOnMainThreadWhenConditionIsMet(

[gab, 2017/04/03 18:20:30]

I'm no clear on what RegisterCallbackToRunOnMainThreadWhenConditionIsMet() is
pointing at in this example and I'm sure it'll be confusing to avg dev.

[fdoray, 2017/04/03 19:18:32]

Done.

+//        base::Bind(&base::RunLoop::Quit, base::Unretained(&run_loop)));

[gab, 2017/04/03 18:20:30]

QuitClosure

[fdoray, 2017/04/03 19:18:33]

Done.

+//    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()->FlushForTesting();

[gab, 2017/04/03 18:20:30]

// Note: base::SequencedWorkerPool::FlushForTesting() is equivalent but
deprecated.

(to enlighten readers of old code wondering why what's used there isn't listed
here)

[fdoray, 2017/04/03 19:18:33]

Done.

+//
+// 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 +62,6 @@ namespace base {
 class MessageLoop;
 namespace test {
 class ScopedAsyncTaskScheduler;
-class ScopedTaskScheduler;
 }  // namespace test
 }  // namespace base
 
@@ -67,8 +80,7 @@ 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 +98,6 @@ 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_;