GN: BUILD.gn for system_wrappers

Also cleaned up some unneeded stuff from webrtc/base/BUILD.gn

BUG=3441
TEST=
Successful compilation of WebRTC as standalone:
gn gen out/Default --args="build_with_chromium=false" && ninja -C out/Default
gn gen out/Default --args="build_with_chromium=false is_clang=true clang_use_chrome_plugins=false" && ninja -C out/Default

I built successfully from a Chromium checkout (with
https://codereview.chromium.org/321313006/ applied) using:
gn gen out/Default && ninja -C out/Default webrtc

R=brettw@chromium.org
TBR=niklas.enbom@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/20739004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6515 4adac7df-926f-26a2-2b94-8c16560cd09d