GN: Replace mojo/system.gni with forwarding groups

mojo/examples/sample_app/BUILD.gn

Index: mojo/examples/sample_app/BUILD.gn
diff --git a/mojo/examples/sample_app/BUILD.gn b/mojo/examples/sample_app/BUILD.gn
index cbbb1f099fe0ab86b0eaf077896cf810e3a06fee..ff2f151f38b61f8bc7741e53401b279b2bb55af5 100644
--- a/mojo/examples/sample_app/BUILD.gn
+++ b/mojo/examples/sample_app/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_sample_app
 shared_library("sample_app") {
   output_name = "mojo_sample_app"
@@ -17,12 +15,14 @@ shared_library("sample_app") {
   deps = [
     ":spinning_cube",
     "//gpu/command_buffer/client:gles2_interface",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
+    "//mojo/public/gles2:for_shared_library",
     "//mojo/services/public/interfaces/geometry",
     "//mojo/services/public/interfaces/native_viewport",
-  ] + mojo_system_for_shared_library + mojo_gles2_for_shared_library
+  ]
 }
 
 source_set("spinning_cube") {
@@ -33,5 +33,6 @@ source_set("spinning_cube") {
 
   deps = [
     "//base",
-  ] + mojo_gles2_for_shared_library
+    "//mojo/public/gles2:for_shared_library",
+  ]
 }