GN: Replace mojo/system.gni with forwarding groups

mojo/examples/echo/BUILD.gn

Index: mojo/examples/echo/BUILD.gn
diff --git a/mojo/examples/echo/BUILD.gn b/mojo/examples/echo/BUILD.gn
index b1e79b6860ee76996a6b9b7f2d22f04b9c6391f7..0f518b4f2694d007252be488f3088a8296f70ed2 100644
--- a/mojo/examples/echo/BUILD.gn
+++ b/mojo/examples/echo/BUILD.gn
@@ -3,7 +3,6 @@
 # found in the LICENSE file.
 
 import("//mojo/public/tools/bindings/mojom.gni")
-import("//mojo/system.gni")
 
 group("echo") {
   deps = [
@@ -18,11 +17,11 @@ shared_library("client") {
 
   deps = [
     ":bindings",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
   ]
-  deps += mojo_system_for_shared_library
 
   sources = [ "echo_client.cc" ]
 }
@@ -33,11 +32,11 @@ shared_library("service") {
 
   deps = [
     ":bindings",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
   ]
-  deps += mojo_system_for_shared_library
 
   sources = [ "echo_service.cc" ]
 }