GN: Replace mojo/system.gni with forwarding groups

mojo/examples/surfaces_app/BUILD.gn

Index: mojo/examples/surfaces_app/BUILD.gn
diff --git a/mojo/examples/surfaces_app/BUILD.gn b/mojo/examples/surfaces_app/BUILD.gn
index 5d7a696dc790fae12ad3296021bbf38423b6bb55..32eeb4d1ba13b9a2f782813b06d4d03961b3e0cc 100644
--- a/mojo/examples/surfaces_app/BUILD.gn
+++ b/mojo/examples/surfaces_app/BUILD.gn
@@ -3,7 +3,6 @@
 # found in the LICENSE file.
 
 import("//mojo/public/tools/bindings/mojom.gni")
-import("//mojo/system.gni")
 
 group("surfaces_app") {
   deps = [
@@ -29,11 +28,12 @@ shared_library("parent_app") {
     "//mojo/application",
     "//mojo/common",
     "//mojo/environment:chromium",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/services/public/interfaces/geometry",
     "//mojo/services/public/cpp/geometry",
     "//mojo/services/public/interfaces/surfaces",
     "//mojo/services/public/cpp/surfaces",
-  ] + mojo_system_for_shared_library
+  ]
 
   sources = [
     "embedder.cc",
@@ -59,11 +59,12 @@ shared_library("child_app") {
     "//mojo/application",
     "//mojo/common",
     "//mojo/environment:chromium",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/services/public/interfaces/geometry",
     "//mojo/services/public/cpp/geometry",
     "//mojo/services/public/interfaces/surfaces",
     "//mojo/services/public/cpp/surfaces",
-  ] + mojo_system_for_shared_library
+  ]
 
   sources = [
     "child_app.cc",
@@ -89,11 +90,13 @@ shared_library("child_gl_app") {
     "//mojo/application",
     "//mojo/environment:chromium",
     "//mojo/examples/sample_app:spinning_cube",
+    "//mojo/public/c/system:for_shared_library",
+    "//mojo/public/gles2:for_shared_library",
     "//mojo/services/public/interfaces/geometry",
     "//mojo/services/public/cpp/geometry",
     "//mojo/services/public/interfaces/surfaces",
     "//mojo/services/public/cpp/surfaces",
-  ] + mojo_system_for_shared_library + mojo_gles2_for_shared_library
+  ]
 
   sources = [
     "child_gl_app.cc",