GN: Replace mojo/system.gni with forwarding groups

mojo/services/html_viewer/BUILD.gn

Index: mojo/services/html_viewer/BUILD.gn
diff --git a/mojo/services/html_viewer/BUILD.gn b/mojo/services/html_viewer/BUILD.gn
index 90e8053667c765f8c57555737cd043a313f56a08..3e640b9bccf8f0163498017f50dbfcc3d36d1ffd 100644
--- a/mojo/services/html_viewer/BUILD.gn
+++ b/mojo/services/html_viewer/BUILD.gn
@@ -2,8 +2,6 @@
 # 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"
@@ -51,6 +49,7 @@ shared_library("html_viewer") {
     "//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",
@@ -64,5 +63,5 @@ shared_library("html_viewer") {
     "//third_party/WebKit/public:blink",
     "//ui/native_theme",
     "//url",
-  ] + mojo_system_for_shared_library
+  ]
 }