Convert third_party source sets to static libraries.

third_party/libwebm/BUILD.gn

 # Copyright 2015 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.
 
 config("libwebm_config") {
   include_dirs = [ "./source" ]
 }
 
-source_set("libwebm") {
+static_library("libwebm") {
   configs += [ ":libwebm_config" ]
   public_configs = [ ":libwebm_config" ]
 
   if (!is_win) {
     cflags = [ "-Wno-deprecated-declarations" ]  # libwebm uses std::auto_ptr
   }
 
   sources = [
     "source/mkvmuxer/mkvmuxer.cc",
     "source/mkvmuxer/mkvmuxerutil.cc",
     "source/mkvmuxer/mkvwriter.cc",
   ]
 }