Arch uses python->python3, not python2. This is fine according to PEP 394, but naturally not everyone is compliant.
We can use the correct python with gn with gn --script-executable=python2.
Theoretically this can go in the .gn file also, but since the official python.org Windows
packages are presently incompatible PEP 394, it's not a slam-dunk.
Arch uses ncurses-6, so we don't have this lib from ncurses-5. We can't use system clang yet, because (at least) ANGLE wants clang-6, and only clang-5 has been released.
You will need: https://aur.archlinux.org/packages/libtinfo5/
Issue: https://bugs.chromium.org/p/chromium/issues/detail?id=777069
Potential cset: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/746142