GN: Fix compile errors with os==chromeos mojo/public build

mojo/public/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 group("public") {
+  # Meta-target, don't link into production code.
+  testonly = true
   deps = [
-    "//mojo/public/c/system",
-    "//mojo/public/cpp/application:standalone",
-    "//mojo/public/cpp/bindings",
-    "//mojo/public/cpp/environment:standalone",
-    "//mojo/public/cpp/utility",
+    ":sdk",
     "//mojo/public/interfaces/bindings/tests:test_interfaces",
-    "//mojo/public/interfaces/application",
-    "//mojo/public/js/bindings",
   ]
 
   if (is_linux) {
     deps += [
       "//mojo/public/python",
     ]
   }
 
   if (is_android) {
     deps += [
       "//mojo/public/java:system",
       "//mojo/public/java:bindings",
     ]
   }
 }
+
+group("sdk") {
+  deps = [
+    "//mojo/public/c/system",
+    "//mojo/public/cpp/application:standalone",
+    "//mojo/public/cpp/bindings",
+    "//mojo/public/cpp/environment:standalone",
+    "//mojo/public/cpp/utility",
+    "//mojo/public/interfaces/application",
+    "//mojo/public/js/bindings",
+  ]
+}
+
+static_library("libmojo_sdk") {
+  complete_static_lib = true
+  deps = [ ":sdk" ]
+}