gn: Reuse IsAsciiAlpha() function from base.

tools/gn/tokenizer.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_TOKENIZER_H_
 #define TOOLS_GN_TOKENIZER_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 15 matching lines @@
   //
   // This is a helper function for error output so that the tokenizer's
   // notion of lines can be used elsewhere.
   static size_t ByteOffsetOfNthLine(const base::StringPiece& buf, int n);
 
   // Returns true if the given offset of the string piece counts as a newline.
   // The offset must be in the buffer.
   static bool IsNewline(const base::StringPiece& buffer, size_t offset);
 
   static bool IsIdentifierFirstChar(char c) {
-    return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || c == '_';
+    return IsAsciiAlpha(c) || c == '_';
   }
 
   static bool IsIdentifierContinuingChar(char c) {
     // Also allow digits after the first char.
     return IsIdentifierFirstChar(c) || IsAsciiDigit(c);
   }
 
  private:
   // InputFile must outlive the tokenizer and all generated tokens.
   explicit Tokenizer(const InputFile* input_file, Err* err);
@@ skipping 33 matching lines @@
   Err* err_;
   size_t cur_;  // Byte offset into input buffer.
 
   int line_number_;
   int char_in_line_;
 
   DISALLOW_COPY_AND_ASSIGN(Tokenizer);
 };
 
 #endif  // TOOLS_GN_TOKENIZER_H_