Mojo: Move waiting APIs to public library

mojo/public/c/system/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.
 
 component("system") {
   output_name = "mojo_public_system"
 
   sources = [
     "buffer.h",
     "core.h",
     "data_pipe.h",
     "functions.h",
     "macros.h",
     "message_pipe.h",
     "platform_handle.h",
     "system_export.h",
     "thunks.cc",
     "thunks.h",
     "types.h",
-    "wait_set.h",
     "watcher.h",
   ]
 
   defines = [ "MOJO_SYSTEM_IMPLEMENTATION" ]
 }
 
 # This should ONLY be depended upon directly by shared_library targets which
 # need to export the MojoSetSystemThunks symbol, like targets generated by the
 # mojo_native_application template in //services/service_manager/public/cpp/service.gni.
 source_set("set_thunks_for_app") {
   sources = [
     "set_thunks_for_app.cc",
   ]
 
   public_deps = [
     ":system",
   ]
 }