Update the gen command to not fail on generator flags.

mojom/mojom_parser/gen_cmd.go

 // Copyright 2016 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.
 
 // This file contains genCmd which parses a set of .mojom files and runs the
 // code generators in order to emit generated bindings for the provided files.
 //
 // The gen command is invoked as follows:
 //
 //      mojom gen [-I <include_dirs>]
@@ skipping 79 matching lines @@
                         "[--src-root-path <path_to_src_root>] "+
                         "[--generators <generator_list>] "+
                         "[--no-gen-imports] "+
                         "<mojom_file>\n\n", filepath.Base(args[0]))
                 fmt.Fprintf(os.Stderr, UsageString(flagSet))
                 fmt.Fprintf(os.Stderr, "Arguments with the --gen-arg- prefix will be passed "+
                         "to every generator. So for instance, to pass --arg=value, you would "+
                         "use --gen-arg-arg=value. To pass --arg, you would use --gen-arg-arg.\n")
         }
 
-        if err := flagSet.Parse(args[2:]); err != nil {
+        // If err is not ErrHelp, the only way to figure out what it was is to look at
+        // the message provided for users.

[vardhan, 2016/04/15 22:53:35]

that's unfortunate

+        if err := flagSet.Parse(args[2:]); err != nil && !strings.HasPrefix(err.Error(), "flag provided but not defined") {
                 if err != flag.ErrHelp {
                         fmt.Fprintln(os.Stderr, err.Error())
                 }
                 printUsage()
                 os.Exit(1)
         }
         generatorPaths := findGeneratorPaths(mojomGenPaths(), generatorNames)
 
         var fileNames []string
         var extraArgs []string
@@ skipping 119 matching lines @@
 // is found, the empty string is returned.
 func findGeneratorPath(paths []string, generatorName string) string {
         for _, path := range paths {
                 genPath := filepath.Join(path, generatorName)
                 if fileInfo, err := os.Stat(genPath); err == nil && !fileInfo.IsDir() {
                         return genPath
                 }
         }
         return ""
 }