Remove header dependencies from sequence_checker.h to sequenced_worker_pool.h

ios/chrome/browser/web_resource/web_resource_util_unittest.cc

 // 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.
 
 #include "ios/chrome/browser/web_resource/web_resource_util.h"
 
 #include <string>
 #include <utility>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/stringprintf.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "base/values.h"
 #include "ios/web/public/web_thread.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace web_resource {
 
 class WebResourceUtilTest : public testing::Test {
  protected:
   WebResourceUtilTest() : error_called_(false), success_called_(false) {}
   ~WebResourceUtilTest() override {}
@@ skipping 94 matching lines @@
 
   FlushBackgroundTasks();
 
   // The error callback is called.
   EXPECT_TRUE(error_called_);
   EXPECT_FALSE(success_called_);
   EXPECT_FALSE(error_.empty());
 }
 
 }  // namespace web_resource