gcc 4.8 does not support c++14 so override the default
c++14 that chromium is now set to.

TBR=kjellander@chromium.org
Bug: libyuv:734
Test: gn gen out/Release "--args=is_debug=false is_clang=false use_sysroot=false target_cpu=\"x64\""
Change-Id: Ic414c79fee90e89ee3539fd1a373da260aecdc2b
Reviewed-on: https://chromium-review.googlesource.com/668031Reviewed-by: Frank Barchard <fbarchard@google.com>
Reviewed-by: Henrik Kjellander <kjellander@chromium.org>