Introduce cc StructTraits unit tests

cc/ipc/BUILD.gn

 # Copyright 2016 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")
 
 component("ipc") {
   output_name = "cc_ipc"
 
   defines = [ "CC_IPC_IMPLEMENTATION" ]
@@ skipping 22 matching lines @@
     "//ui/gfx/ipc/skia",
   ]
 }
 
 mojom("interfaces") {
   sources = [
     "render_pass_id.mojom",
     "surface_id.mojom",
   ]
 }
+
+mojom("test_interfaces") {
+  testonly = true
+  sources = [
+    "traits_test_service.mojom",
+  ]
+
+  public_deps = [
+    ":interfaces",
+  ]
+}