
Closes https://github.com/flutter/flutter/issues/156909. This PR adds (and implements) the `--gn-args` (extra command-line GN args) functionality by generalizing on the concept of "merged" GN args that @zanderso had special-cased for `--lto` and `--rbe`, and further testing it. There is also a logical place for us to expand support of merged arguments at a future point in time.