GN: Replace mojo/system.gni with forwarding groups

mojo/examples/png_viewer/BUILD.gn

Index: mojo/examples/png_viewer/BUILD.gn
diff --git a/mojo/examples/png_viewer/BUILD.gn b/mojo/examples/png_viewer/BUILD.gn
index 7122a06f8599942852f1a0d80518ffba2e7fbd22..3fc0360e2398a62e0e7fce1f1f6f81302657c74a 100644
--- a/mojo/examples/png_viewer/BUILD.gn
+++ b/mojo/examples/png_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_examples.gypi:mojo_png_viewer
 shared_library("png_viewer") {
   output_name = "mojo_png_viewer"
@@ -15,6 +13,7 @@ shared_library("png_viewer") {
   deps = [
     "//mojo/application",
     "//mojo/examples/media_viewer:bindings",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
     "//mojo/services/public/cpp/view_manager",
@@ -22,5 +21,5 @@ shared_library("png_viewer") {
     "//mojo/services/public/interfaces/network",
     "//skia",
     "//ui/gfx",
-  ] + mojo_system_for_shared_library
+  ]
 }