gn: Reuse IsAsciiAlpha() function from base.

tools/gn/ninja_toolchain_writer.h

 // 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.
 
 #ifndef TOOLS_GN_NINJA_TOOLCHAIN_WRITER_H_
 #define TOOLS_GN_NINJA_TOOLCHAIN_WRITER_H_
 
 #include <iosfwd>
 #include <set>
 #include <string>
 #include <vector>
 
 #include "tools/gn/ninja_helper.h"
 #include "tools/gn/path_output.h"
 
 class BuildSettings;
 class Settings;
 class Target;
 class Toolchain;
 
 class NinjaToolchainWriter {
  public:
   // Takes the settings for the toolchain, as well as the list of all targets
-  // assicoated with the toolchain.
+  // associated with the toolchain.
   static bool RunAndWriteFile(const Settings* settings,
                               const Toolchain* toolchain,
                               const std::vector<const Target*>& targets);
 
  private:
   NinjaToolchainWriter(const Settings* settings,
                        const Toolchain* toolchain,
                        const std::vector<const Target*>& targets,
                        std::ostream& out);
   ~NinjaToolchainWriter();
 
   void Run();
 
   void WriteRules();
   void WriteSubninjas();
 
   const Settings* settings_;
   const Toolchain* toolchain_;
   std::vector<const Target*> targets_;
   std::ostream& out_;
   PathOutput path_output_;
 
   NinjaHelper helper_;
 
   DISALLOW_COPY_AND_ASSIGN(NinjaToolchainWriter);
 };
 
 #endif  // TOOLS_GN_NINJA_TOOLCHAIN_WRITER_H_