Convert third_party source sets to static libraries.

base/third_party/libevent/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/nacl/config.gni")
 
-source_set("libevent") {
+static_library("libevent") {
   sources = [
     "buffer.c",
     "evbuffer.c",
     "evdns.c",
     "event.c",
     "event_tagging.c",
     "evrpc.c",
     "evutil.c",
     "http.c",
     "log.c",
@@ skipping 28 matching lines @@
       "nacl_nonsfi/event-config.h",
       "nacl_nonsfi/random.c",
       "nacl_nonsfi/signal_stub.c",
     ]
     include_dirs = [ "nacl_nonsfi" ]
   }
 
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
 }