Make mojo::RunLoop just use pthreads.

mojo/public/cpp/utility/BUILD.gn

 # Copyright 2014 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("../../mojo_sdk.gni")
 
 mojo_sdk_source_set("utility") {
   sources = [
     "lib/run_loop.cc",
-    "lib/thread_local.h",
-    "lib/thread_local_posix.cc",
-    "lib/thread_local_win.cc",
     "run_loop.h",
     "run_loop_handler.h",
   ]
 
   mojo_sdk_deps = [
     "mojo/public/cpp/bindings:callback",
     "mojo/public/cpp/system",
   ]
 }