GN: Replace mojo/system.gni with forwarding groups

mojo/examples/content_handler_demo/BUILD.gn

Index: mojo/examples/content_handler_demo/BUILD.gn
diff --git a/mojo/examples/content_handler_demo/BUILD.gn b/mojo/examples/content_handler_demo/BUILD.gn
index 0fe1aef19306a06f6b54eb2a1b1f6b0492378931..cd8bcbbb08269bbeabd5a3c8e996790a65c1c713 100644
--- a/mojo/examples/content_handler_demo/BUILD.gn
+++ b/mojo/examples/content_handler_demo/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_examples.gypi:mojo_content_handler_demo
 shared_library("content_handler_demo") {
   output_name = "mojo_content_handler_demo"
@@ -14,9 +12,10 @@ shared_library("content_handler_demo") {
 
   deps = [
     "//mojo/application",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
     "//mojo/services/public/interfaces/content_handler",
-  ] + mojo_system_for_shared_library
+  ]
 }