Change the static_library("webrtc") to a source set in the GN build.
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: 03ae4fbe531b1eefc9d815f31e49022782c42458@7687 4adac7df-926f-26a2-2b94-8c16560cd09d

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