GN: Replace mojo/system.gni with forwarding groups

mojo/examples/window_manager/BUILD.gn

Index: mojo/examples/window_manager/BUILD.gn
diff --git a/mojo/examples/window_manager/BUILD.gn b/mojo/examples/window_manager/BUILD.gn
index 1f6054ff6379794d3618e28f7c23563834c1b7c2..2d55368b8c8b5ecf8d95b48be775b133a3b3960f 100644
--- a/mojo/examples/window_manager/BUILD.gn
+++ b/mojo/examples/window_manager/BUILD.gn
@@ -3,7 +3,6 @@
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
-import("//mojo/system.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 
 assert(use_aura)
@@ -24,8 +23,10 @@ shared_library("window_manager") {
     "//mojo/application",
     "//mojo/aura",
     "//mojo/examples/keyboard:bindings",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
+    "//mojo/public/gles2:for_shared_library",
     "//mojo/services/public/cpp/geometry",
     "//mojo/services/public/cpp/input_events",
     "//mojo/services/public/cpp/view_manager",
@@ -41,7 +42,7 @@ shared_library("window_manager") {
     "//ui/gl",
     "//ui/resources",
     "//ui/views",
-  ] + mojo_system_for_shared_library + mojo_gles2_for_shared_library
+  ]
 }
 
 # GYP version: mojo/mojo_examples.gypi:mojo_window_manager_bindings