Remove header dependencies from sequence_checker.h to sequenced_worker_pool.h

ios/chrome/browser/snapshots/snapshot_cache_unittest.mm

 // Copyright 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.
 
 #import "ios/chrome/browser/snapshots/snapshot_cache.h"
 
 #import <Foundation/Foundation.h>
 
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/format_macros.h"
 #include "base/location.h"
 #include "base/mac/bind_objc_block.h"
 #include "base/mac/scoped_cftyperef.h"
 #include "base/mac/scoped_nsautorelease_pool.h"
 #include "base/mac/scoped_nsobject.h"
 #include "base/run_loop.h"
 #include "base/strings/sys_string_conversions.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "base/time/time.h"
 #import "ios/chrome/browser/snapshots/snapshot_cache_internal.h"
 #include "ios/web/public/test/test_web_thread_bundle.h"
 #include "ios/web/public/web_thread.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/gtest_mac.h"
 #include "testing/platform_test.h"
 
 static const NSUInteger kSessionCount = 10;
 static const NSUInteger kSnapshotPixelSize = 8;
@@ skipping 534 matching lines @@
   EXPECT_TRUE(base::PathExists(retinaFile));
 
   // Delete the image.
   [cache removeImageWithSessionID:kSession];
   FlushRunLoops();  // ensure the file is removed.
 
   EXPECT_FALSE(base::PathExists(retinaFile));
 }
 
 }  // namespace