The latest GN binaries rename cpu_arch to current_cpu and/or target_cpu
as appropriate, build_cpu_arch to host_cpu, and os to current_os and target_os as appropriate.

R=brettw@chromium.org
TBR=ddorwin@chromium.org
EXTRA_TRYBOTS=tryserver.chromium.linux:android_chromium_gn_compile_dbg,android_chromium_gn_compile_rel;tryserver.chromium.win:win8_chromium_gn_rel,win8_chromium_gn_dbg;tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg
BUG=344767

Review URL: https://codereview.chromium.org/913373002

Cr-Commit-Position: refs/heads/master@{#317223}