Add spell-checking to `gn help`.

tools/gn/string_utils.h

Index: tools/gn/string_utils.h
diff --git a/tools/gn/string_utils.h b/tools/gn/string_utils.h
index 07be4c856e4708cda02b6a7fd5aa466f0dc3a03d..744714a754a35b674ffb6e432562774a4ac2b962 100644
--- a/tools/gn/string_utils.h
+++ b/tools/gn/string_utils.h
@@ -5,6 +5,8 @@
 #ifndef TOOLS_GN_STRING_UTILS_H_
 #define TOOLS_GN_STRING_UTILS_H_
 
+#include <vector>
+
 #include "base/strings/string_piece.h"
 
 class Err;
@@ -35,4 +37,17 @@ bool ExpandStringLiteral(Scope* scope,
                          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_