PSA: Remember to pass either is_official_build=true or dcheck_always_on=false to GN

[Kubo Da Costa, Raphael]

This was announced a while ago in chromium-dev, but it's also relevant
to this list:

https://groups.google.com/a/chromium.org/g/chromium-dev/c/cIlKuPp9aGk/m/VsGgYsYqAgAJ

In summary, since M94 if you're not passing is_official_build=true to
GN, the dcheck_always_on variable will end up defaulting to true, which
means users are more likely to hit crashes and the browser will run
more slowly (see the discussion in
https://bugs.chromium.org/p/chromium/issues/detail?id=1221442#c48)

As the chromium-dev thread indicates, you should probably be building
your packages with is_official_build=true, but if for whatever reason
you don't, make sure you at least pass dcheck_always_on=false
explicitly.