In  Issue 758630  the ability to inject a Swarming trigger script was added, in order to customize how jobs are triggered in the Swarming pool.

The mechanism is very useful, but the --swarming-py-path argument that is passed to the trigger script is highly problematic. Here's why:

1) The trigger script absolutely has to filter out this argument before turning around and invoking "swarming.py trigger". Otherwise, the target script or executable will see it and be confused by it.

2) The easiest way to do this is to use argparse to add an argument "--swarming-py-path", and use ArgumentParser.parse_known_args() to chomp this and other expected arguments before calling "swarming.py trigger".

3) Unfortunately, ArgumentParser has built-in functionality to allow arguments to be abbreviated. --swarming-py-path therefore collides with the necessary argument "--swarming" which is passed to swarming.py. This ends up chomping the wrong command line argument.

4) Adding another argument to argparse "--swarming" does not really help, because it turns out that "--swarming-py-path" is placed after the "--" in the command line, and this causes argparse to not consume it, anyway.

The best solution is for recipe_modules/swarming/api.py  to not pass this argument to the trigger script at all. It's unnecessary, because the trigger script is invoked out of the Chromium checkout, and the script can figure out where src/tools/swarming_client lives relative to it and just invoke it itself.