Revert of Revert "Allow SequencedTaskRunnerHandle::Get() while running unsequenced tasks."

base/threading/sequenced_task_runner_handle.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_THREADING_SEQUENCED_TASK_RUNNER_HANDLE_H_
 #define BASE_THREADING_SEQUENCED_TASK_RUNNER_HANDLE_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/sequenced_task_runner.h"
 
 namespace base {
 
 class BASE_EXPORT SequencedTaskRunnerHandle {
  public:
   // Returns a SequencedTaskRunner which guarantees that posted tasks will only
   // run after the current task is finished and will satisfy a SequenceChecker.
   // It should only be called if IsSet() returns true (see the comment there for
   // the requirements).
   static scoped_refptr<SequencedTaskRunner> Get();
 
   // Returns true if one of the following conditions is fulfilled:
   // a) A SequencedTaskRunner has been assigned to the current thread by
   //    instantiating a SequencedTaskRunnerHandle.
   // b) The current thread has a ThreadTaskRunnerHandle (which includes any
   //    thread that has a MessageLoop associated with it), or
-  // c) The current thread is a worker thread belonging to a SequencedWorkerPool
-  //    *and* is currently running a sequenced task (note: not supporting
-  //    unsequenced tasks is intentional: https://crbug.com/618043#c4).
+  // c) The current thread is a worker thread belonging to a
+  //    SequencedWorkerPool.
   static bool IsSet();
 
   // Binds |task_runner| to the current thread.
   explicit SequencedTaskRunnerHandle(
       scoped_refptr<SequencedTaskRunner> task_runner);
   ~SequencedTaskRunnerHandle();
 
  private:
   scoped_refptr<SequencedTaskRunner> task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(SequencedTaskRunnerHandle);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_SEQUENCED_TASK_RUNNER_HANDLE_H_