Add spell-checking to `gn help`.

tools/gn/string_utils_unittest.cc

Index: tools/gn/string_utils_unittest.cc
diff --git a/tools/gn/string_utils_unittest.cc b/tools/gn/string_utils_unittest.cc
index d62e17e7b81055cf848f8b75a90bb624bfc6ba5b..570997ff1b72bd2f1acbb6c60e9e946dc8444298 100644
--- a/tools/gn/string_utils_unittest.cc
+++ b/tools/gn/string_utils_unittest.cc
@@ -111,3 +111,44 @@ TEST(StringUtils, ExpandStringLiteralExpression) {
   EXPECT_TRUE(CheckExpansionCase("${1 + 2}", nullptr, false));
   EXPECT_TRUE(CheckExpansionCase("${print(1)}", nullptr, false));
 }
+
+TEST(StringUtils, EditDistance) {
+  EXPECT_EQ(3u, EditDistance("doom melon", "dune melon", 100));
+  EXPECT_EQ(2u, EditDistance("doom melon", "dune melon", 1));
+
+  EXPECT_EQ(2u, EditDistance("ab", "ba", 100));
+  EXPECT_EQ(2u, EditDistance("ba", "ab", 100));
+
+  EXPECT_EQ(2u, EditDistance("ananas", "banana", 100));
+  EXPECT_EQ(2u, EditDistance("banana", "ananas", 100));
+
+  EXPECT_EQ(2u, EditDistance("unclear", "nuclear", 100));
+  EXPECT_EQ(2u, EditDistance("nuclear", "unclear", 100));
+
+  EXPECT_EQ(3u, EditDistance("chrome", "chromium", 100));
+  EXPECT_EQ(3u, EditDistance("chromium", "chrome", 100));
+
+  EXPECT_EQ(4u, EditDistance("", "abcd", 100));
+  EXPECT_EQ(4u, EditDistance("abcd", "", 100));
+
+  EXPECT_EQ(4u, EditDistance("xxx", "xxxxxxx", 100));
+  EXPECT_EQ(4u, EditDistance("xxxxxxx", "xxx", 100));
+
+  EXPECT_EQ(7u, EditDistance("yyy", "xxxxxxx", 100));
+  EXPECT_EQ(7u, EditDistance("xxxxxxx", "yyy", 100));
+}
+
+TEST(StringUtils, SpellcheckString) {
+  std::vector<base::StringPiece> words;
+  words.push_back("your");
+  words.push_back("bravado");
+  words.push_back("won\'t");
+  words.push_back("help");
+  words.push_back("you");
+  words.push_back("now");
+
+  EXPECT_EQ("help", SpellcheckString("halp", words));
+
+  // barbados has an edit distance of 4 from bravado, so there's no suggestion.
+  EXPECT_TRUE(SpellcheckString("barbados", words).empty());
+}