Terminal Chromium like build environment

To run gn, ninja, siso and other well-known Chromium tools directly in the current terminal session use helpers located in brave/build directory:

# cygwin/linux/macos
. brave/build/env.sh

# windows powershell
. brave\build\env.ps1

# windows cmd.exe
brave\build\env.cmd

Pass -v to see what variables are set.