devel/chromium-gn: Add port to the tree
GN is a meta-build system that generates NinjaBuild.
It helps to build chromium with Ninja.
Also enables us for one step forward to build Electron.io for FreeBSD.

Reported by: Luca Pizzamiglio <luca.pizzamiglio@gmail.com>
Reviewed by: mat (mentor), mmokhi, cpm (chromium)
Approved by: mat (mentor), mmokhi, cpm (chromium)
Differential Revision: https://reviews.freebsd.org/D11554