Revert of Disallow posting tasks to SequencedWorkerPools by default.

base/threading/sequenced_worker_pool.h

Index: base/threading/sequenced_worker_pool.h
diff --git a/base/threading/sequenced_worker_pool.h b/base/threading/sequenced_worker_pool.h
index f1070d573b4c36c12fc5b4d731e074ffe96a0736..252d511d3f2bae0f09738c0939d0d2932dc10877 100644
--- a/base/threading/sequenced_worker_pool.h
+++ b/base/threading/sequenced_worker_pool.h
@@ -58,10 +58,6 @@
 // You can also post tasks to the pool without ordering using PostWorkerTask.
 // These will be executed in an unspecified order. The order of execution
 // between tasks with different sequence tokens is also unspecified.
-//
-// You must call EnableForProcess() or
-// EnableWithRedirectionToTaskSchedulerForProcess() before starting to post
-// tasks to a process' SequencedWorkerPools.
 //
 // This class may be leaked on shutdown to facilitate fast shutdown. The
 // expected usage, however, is to call Shutdown(), which correctly accounts
@@ -182,27 +178,25 @@
   // PostSequencedWorkerTask(). Valid tokens are always nonzero.
   static SequenceToken GetSequenceToken();
 
-  // Enables posting tasks to this process' SequencedWorkerPools. Cannot be
-  // called if already enabled. This is not thread-safe; proper synchronization
-  // is required to use any SequencedWorkerPool method after calling this.
-  static void EnableForProcess();
-
-  // Same as EnableForProcess(), but tasks are redirected to the registered
-  // TaskScheduler. There must be a registered TaskScheduler when this is
-  // called.
+  // Starts redirecting tasks posted to this process' SequencedWorkerPools to
+  // the registered TaskScheduler. This cannot be called after a task has been
+  // posted to a SequencedWorkerPool. This is not thread-safe; proper
+  // synchronization is required to use any SequencedWorkerPool method after
+  // calling this. There must be a registered TaskScheduler when this is called.
+  // Ideally, call this on the main thread of a process, before any other
+  // threads are created and before any tasks are posted to that process'
+  // SequencedWorkerPools.
   // TODO(gab): Remove this if http://crbug.com/622400 fails
   // (SequencedWorkerPool will be phased out completely otherwise).
-  static void EnableWithRedirectionToTaskSchedulerForProcess();
-
-  // Disables posting tasks to this process' SequencedWorkerPools. Calling this
-  // while there are active SequencedWorkerPools is not supported. This is not
-  // thread-safe; proper synchronization is required to use any
-  // SequencedWorkerPool method after calling this.
-  static void DisableForProcessForTesting();
-
-  // Returns true if posting tasks to this process' SequencedWorkerPool is
-  // enabled (with or without redirection to TaskScheduler).
-  static bool IsEnabled();
+  static void RedirectToTaskSchedulerForProcess();
+
+  // Stops redirecting tasks posted to this process' SequencedWorkerPools to the
+  // registered TaskScheduler and allows RedirectToTaskSchedulerForProcess() to
+  // be called even if tasks have already posted to a SequencedWorkerPool in
+  // this process. Calling this while there are active SequencedWorkerPools is
+  // not supported. This is not thread-safe; proper synchronization is required
+  // to use any SequencedWorkerPool method after calling this.
+  static void ResetRedirectToTaskSchedulerForProcessForTesting();
 
   // When constructing a SequencedWorkerPool, there must be a
   // ThreadTaskRunnerHandle on the current thread unless you plan to