Move ReadFileToString to the base namespace.

tools/gn/input_file.cc

 // Copyright (c) 2013 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 "tools/gn/input_file.h"
 
 #include "base/file_util.h"
 
 InputFile::InputFile(const SourceFile& name)
     : name_(name),
       dir_(name_.GetDir()),
       contents_loaded_(false) {
 }
 
 InputFile::~InputFile() {
 }
 
 void InputFile::SetContents(const std::string& c) {
   contents_loaded_ = true;
   contents_ = c;
 }
 
 bool InputFile::Load(const base::FilePath& system_path) {
-  if (file_util::ReadFileToString(system_path, &contents_)) {
+  if (base::ReadFileToString(system_path, &contents_)) {
     contents_loaded_ = true;
     physical_name_ = system_path;
     return true;
   }
   return false;
 }