Change the static_library("webrtc") to a source set in the GN build.

Static libraries cannot have only headers as sources (libtool complains
that there's nothing to actually link).

TBR=brettw@chromium.org

Review URL: https://webrtc-codereview.appspot.com/27159004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7687 4adac7df-926f-26a2-2b94-8c16560cd09d