tools/gn: fix escaping of backslashes

tools/gn/value.cc

Index: tools/gn/value.cc
diff --git a/tools/gn/value.cc b/tools/gn/value.cc
index 6dfe23417a4a4d6ee3183e613561bc7f434b1c18..687c0396db325e551edb43781efe9ae1ff5cf185 100644
--- a/tools/gn/value.cc
+++ b/tools/gn/value.cc
@@ -124,15 +124,20 @@ std::string Value::ToString(bool quote_string) const {
       return base::Int64ToString(int_value_);
     case STRING:
       if (quote_string) {
-        std::string escaped = string_value_;
-        // First escape all special uses of a backslash.
-        ReplaceSubstringsAfterOffset(&escaped, 0, "\\$", "\\\\$");
-        ReplaceSubstringsAfterOffset(&escaped, 0, "\\\"", "\\\\\"");
-
-        // Now escape special chars.
-        ReplaceSubstringsAfterOffset(&escaped, 0, "$", "\\$");
-        ReplaceSubstringsAfterOffset(&escaped, 0, "\"", "\\\"");
-        return "\"" + escaped + "\"";
+        std::string result = "\"";
+        bool escaped = false;

[brettw, 2015/03/31 23:01:51]

The goal of tracking this should be documented, it took me quite a while to
realize you're trying to avoid unnecessarily escaping backslashes.

[mdempsky, 2015/03/31 23:14:47]

Done, PTAL.

+        for (char ch : string_value_) {
+          if (escaped && (ch == '$' || ch == '"' || ch == '\\'))
+            result += '\\';
+          if (ch == '$' || ch == '"')
+            result += '\\';
+          result += ch;
+          escaped = (ch == '\\');
+        }
+        if (escaped)
+          result += '\\';
+        result += '"';
+        return result;
       }
       return string_value_;
     case LIST: {