Remove task.h and finish base::Bind() migration.

base/bind_helpers.cc

 // Copyright (c) 2012 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.
 
-#include "base/task.h"
+#include "base/bind_helpers.h"
+
+#include "base/callback.h"
 
 namespace base {
 
+void DoNothing() {
+}
+
 ScopedClosureRunner::ScopedClosureRunner(const Closure& closure)
     : closure_(closure) {
 }
 
 ScopedClosureRunner::~ScopedClosureRunner() {
   if (!closure_.is_null())
     closure_.Run();
 }
 
 Closure ScopedClosureRunner::Release() {
   Closure result = closure_;
   closure_.Reset();
   return result;
 }
 
 }  // namespace base