Add spell-checking to `gn help`.

tools/gn/string_utils.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_STRING_UTILS_H_
 #define TOOLS_GN_STRING_UTILS_H_
 
+#include <vector>
+
 #include "base/strings/string_piece.h"
 
 class Err;
 class Scope;
 class Token;
 class Value;
 
 inline std::string operator+(const std::string& a, const base::StringPiece& b) {
   std::string ret;
   ret.reserve(a.size() + b.size());
@@ skipping 10 matching lines @@
   return ret;
 }
 
 // Unescapes and expands variables in the given literal, writing the result
 // to the given value. On error, sets |err| and returns false.
 bool ExpandStringLiteral(Scope* scope,
                          const Token& literal,
                          Value* result,
                          Err* err);
 
+// Returns the minimum number of inserts, deleted, and replacements of
+// characters needed to transform s1 to s2, or max_edit_distance + 1 if
+// transforming s1 into s2 isn't possible in at most max_edit_distance steps.
+size_t EditDistance(const base::StringPiece& s1,
+                    const base::StringPiece& s2,
+                    size_t max_edit_distance);
+
+// Given a string |text| and a vector of correctly-spelled strings |words|,
+// returns the first string in |words| closest to |text|, or an empty
+// StringPiece if none of the strings in |words| is close.
+base::StringPiece SpellcheckString(const base::StringPiece& text,
+                                   const std::vector<base::StringPiece>& words);
+
 #endif  // TOOLS_GN_STRING_UTILS_H_