C++ bindings: separate out serialization source set, have "mojom" targets optionally use serializat…

examples/serialization/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.
+
+import("//mojo/public/tools/bindings/mojom.gni")
+
+group("serialization") {
+  deps = [
+    ":serialization_example",
+  ]
+}
+
+executable("serialization_example") {
+  sources = [
+    "main.cc",
+  ]
+
+  deps = [
+    ":serialization_mojom",
+  ]
+}
+
+mojom("serialization_mojom") {
+  sources = [
+    "serialization.mojom",
+  ]
+
+  # By providing the |dataonly| flag, you can prevent the generated mojom build
+  # target from referencing the mojo system primitives.
+  #
+  # This also means that we can't serialize mojom handle types.
+  dataonly = true
+}