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/+/2204627
Reviewed-by: Ian Clelland <[email protected]>
Commit-Queue: Jason Chase <[email protected]>
Cr-Commit-Position: refs/heads/master@{#771081}
diff --git a/tools/origin_trials/generate_token.py b/tools/origin_trials/generate_token.py
index f229a785..befb051 100755
--- a/tools/origin_trials/generate_token.py
+++ b/tools/origin_trials/generate_token.py
@@ -10,7 +10,8 @@
                           --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.
 """
@@ -131,11 +132,10 @@
 
   parser = argparse.ArgumentParser(
       description="Generate tokens for enabling experimental features")
-  parser.add_argument(
-      "version",
-      help="Token version to use. Currently only version 2"
-      "and version 3 are supported.",
-      type=VersionFromArg)
+  parser.add_argument("--version",
+                      help="Token version to use. Currently only version 2"
+                      "and version 3 are supported.",
+                      type=VersionFromArg)
   parser.add_argument("origin",
                       help="Origin for which to enable the feature. This can "
                            "be either a hostname (default scheme HTTPS, "