previously gyp enabled
'chromium_code': 1,
for stricter warnings, but GN disabled warnings:
configs -= [ "//build/config/compiler:chromium_code" ]
configs += [ "//build/config/compiler:no_chromium_code" ]

This CL enables strict warnings for GN as well as GYP.

R=rsesek@chromium.org
BUG=libyuv:617

Review URL: https://codereview.chromium.org/2130583003 .