GN flag to fail for unused build args.

This flag was requested by ChromeOS so their bot can notice if their build's args gets out of sync with Chrome's build files.

Also removes printing all possible build args on this error because we've gotten so many that's it's several screen-fulls of text and you can't even read the error.

BUG=605199

Review URL: https://codereview.chromium.org/1907613002

Cr-Original-Commit-Position: refs/heads/master@{#389407}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 12985c01ad47c1366920941f837fc2f323480c0e
5 files changed
tree: 744db0e5571da607b0429cf6f5121c883a9b2a44
  1. tools/