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 <firstname.lastname@example.org>
Reviewed by: mat (mentor), mmokhi, cpm (chromium)
Approved by: mat (mentor), mmokhi, cpm (chromium)
Differential Revision: https://reviews.freebsd.org/D11554