Improve usability for generate_token.py

Make |version| a named parameter instead of positional. This should
make command lines less error-prone and magical (i.e. "what's this 3 in
the middle of the command").

Change-Id: I7ac59fa0268c0015f99232a8d3e24a64dadee717
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2204627Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Jason Chase <chasej@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771081}

tools/origin_trials/generate_token.py

@@ -10,7 +10,8 @@ usage: generate_token.py [-h] [--key-file KEY_FILE]
                           --expire-timestamp EXPIRE_TIMESTAMP]
                          [--is_subdomain | --no-subdomain]
                          [--is_third-party | --no-third-party]
-                         version origin trial_name
+                         --version=VERSION
+                         origin trial_name
 
 Run "generate_token.py -h" for more help on usage.
 """
@@ -131,8 +132,7 @@ def main():
 
   parser = argparse.ArgumentParser(
       description="Generate tokens for enabling experimental features")
-  parser.add_argument(
-      "version",
+  parser.add_argument("--version",
                       help="Token version to use. Currently only version 2"
                       "and version 3 are supported.",
                       type=VersionFromArg)