GN: Replace mojo/system.gni with forwarding groups

mojo/examples/echo/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/public/tools/bindings/mojom.gni")
-import("//mojo/system.gni")
 
 group("echo") {
   deps = [
     ":client",
     ":service"
   ]
 }
 
 # GYP version: mojo/mojo_examples.gypi:mojo_echo_client
 shared_library("client") {
   output_name = "mojo_echo_client"
 
   deps = [
     ":bindings",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
   ]
-  deps += mojo_system_for_shared_library
 
   sources = [ "echo_client.cc" ]
 }
 
 # GYP version: mojo/mojo_examples.gypi:mojo_echo_service
 shared_library("service") {
   output_name = "mojo_echo_service"
 
   deps = [
     ":bindings",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
   ]
-  deps += mojo_system_for_shared_library
 
   sources = [ "echo_service.cc" ]
 }
 
 # GYP version: mojo/mojo_examples.gypi:mojo_echo_service_bindings
 mojom("bindings") {
   sources = [ "echo_service.mojom" ]
 }