[Sync] Removing duplicated includes between cc and h files.

components/sync/test/fake_server/sessions_hierarchy.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 "components/sync/test/fake_server/sessions_hierarchy.h"
 
 #include <sstream>
-#include <string>
 
 namespace fake_server {
 
 SessionsHierarchy::SessionsHierarchy() {}
 
 SessionsHierarchy::~SessionsHierarchy() {}
 
 void SessionsHierarchy::AddWindow(const std::string& tab) {
   SessionsHierarchy::Window window;
   window.insert(tab);
@@ skipping 24 matching lines @@
   }
   output << "}";
   return output.str();
 }
 
 bool SessionsHierarchy::Equals(const SessionsHierarchy& other) const {
   return windows_ == other.windows_;
 }
 
 }  // namespace fake_server