PSA: Updated command line interface for the mojom tool.

[Alexandre]

I just landed a change (https://codereview.chromium.org/1702873002/) to the mojom parser in order to make it more versatile. It now accepts subcommands a-la git or go tool. Specifically, in order to invoke the parser, you must now run

mojom_parser parse <same args as usual>

This change is to accommodate upcoming extra functionality such as the mojom formatter. You will need to sync to head and run gclient sync to see those changes.

In the near future, we will rename mojom_parser to mojom to reflect the mojom tool's greater range of function.

If this change breaks you and the provided information is not enough to fix your build, please let me know and I will be happy to help.

Alex