Mojo: Move waiting APIs to public library

mojo/public/cpp/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.
 
 # Deletes libsystem.dylib from the build dir, since it shadows
 # /usr/lib/libSystem.dylib on macOS.
 # TODO(thakis): Remove this after a while.
 action("clean_up_old_dylib") {
   script = "//build/rm.py"
   stamp = "$target_gen_dir/clean_up_stamp"
@@ skipping 19 matching lines @@
     "functions.h",
     "handle.h",
     "handle_signals_state.h",
     "message.h",
     "message_pipe.h",
     "platform_handle.cc",
     "platform_handle.h",
     "simple_watcher.cc",
     "simple_watcher.h",
     "system_export.h",
+    "wait.cc",
+    "wait.h",
+    "wait_set.cc",
+    "wait_set.h",
     "watcher.cc",
     "watcher.h",
   ]
 
   public_deps = [
     "//base",
     "//mojo/public/c/system",
   ]
   deps = [
     ":clean_up_old_dylib",
   ]
 
   defines = [ "MOJO_CPP_SYSTEM_IMPLEMENTATION" ]
 }