GN: Replace mojo/system.gni with forwarding groups

mojo/services/html_viewer/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/system.gni")
-
 # GYP version: mojo/mojo_services.gypi:mojo_html_viewer
 shared_library("html_viewer") {
   output_name = "mojo_html_viewer"
 
   sources = [
     "blink_basic_type_converters.cc",
     "blink_basic_type_converters.h",
     "blink_input_events_type_converters.cc",
     "blink_input_events_type_converters.h",
     "blink_platform_impl.cc",
@@ skipping 27 matching lines @@
 
   deps = [
     "//cc",
     "//cc/blink",
     "//cc/surfaces",
     "//media",
     "//media/blink",
     "//mojo/cc",
     "//mojo/common",
     "//mojo/application",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
     "//mojo/services/public/cpp/view_manager",
     "//mojo/services/public/interfaces/content_handler",
     "//mojo/services/public/interfaces/gpu",
     "//mojo/services/public/interfaces/navigation",
     "//mojo/services/public/interfaces/network",
     "//mojo/services/public/interfaces/surfaces",
     "//net",
     "//skia",
     "//third_party/WebKit/public:blink",
     "//ui/native_theme",
     "//url",
-  ] + mojo_system_for_shared_library
+  ]
 }