tools/gn: Give a better informative message if user uses apostrophe.

tools/gn/tokenizer.cc

 // 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.
 
 #include "tools/gn/tokenizer.h"
 
 #include "base/logging.h"
 #include "base/strings/string_util.h"
 #include "tools/gn/input_file.h"
 
@@ skipping 370 matching lines @@
     // Semicolon.
     help = "Semicolons are not needed, delete this one.";
   } else if (cur_char() == '\t') {
     // Tab.
     help = "You got a tab character in here. Tabs are evil. "
            "Convert to spaces.";
   } else if (cur_char() == '/' && cur_ + 1 < input_.size() &&
       (input_[cur_ + 1] == '/' || input_[cur_ + 1] == '*')) {
     // Different types of comments.
     help = "Comments should start with # instead";
+  } else if (cur_char() == '\'') {
+    help = "Strings are delimited by \" characters, not apostrophes.";
   } else {
     help = "I have no idea what this is.";
   }
 
   return Err(location, "Invalid token.", help);
 }