Implement Global Privacy Control (only in nightly)

[pes10k]

Several browsers and publishers are working on a new Global Privacy Control spec, that will be announced next week. The full implementation in Brave would be:

1. add a Sec-GPC header, with the value always being 1
2. add a navigator.globalPrivacyControl end point in the DOM, that always returns true (boolean, not string)

No UX or shields integration is needed. No state is set or maintained.

Ideally it would:

1. always be enabled for nightly builds
2. be default disabled in all other channels
3. have a flag so that non-nightly builds could enable too once this makes way to other channels

3 is not needed though if it makes the implementation more difficult