Replace forward declarations with includes in TaskRunner APIs.

base/threading/thread_task_runner_handle.h

 // Copyright 2016 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_THREAD_TASK_RUNNER_HANDLE_H_
 #define BASE_THREADING_THREAD_TASK_RUNNER_HANDLE_H_
 
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/single_thread_task_runner.h"
 
 namespace base {
 
-class SingleThreadTaskRunner;
-
 // ThreadTaskRunnerHandle stores a reference to a thread's TaskRunner
 // in thread-local storage.  Callers can then retrieve the TaskRunner
 // for the current thread by calling ThreadTaskRunnerHandle::Get().
 // At most one TaskRunner may be bound to each thread at a time.
 // Prefer SequenceTaskRunnerHandle to this unless thread affinity is required.
 class BASE_EXPORT ThreadTaskRunnerHandle {
  public:
   // Gets the SingleThreadTaskRunner for the current thread.
   static scoped_refptr<SingleThreadTaskRunner> Get();
 
   // Returns true if the SingleThreadTaskRunner is already created for
   // the current thread.
   static bool IsSet();
 
   // Binds |task_runner| to the current thread. |task_runner| must belong
   // to the current thread for this to succeed.
   explicit ThreadTaskRunnerHandle(
       scoped_refptr<SingleThreadTaskRunner> task_runner);
   ~ThreadTaskRunnerHandle();
 
  private:
   scoped_refptr<SingleThreadTaskRunner> task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadTaskRunnerHandle);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_TASK_RUNNER_HANDLE_H_