C bindings: Implement _DeepCopy() & some unittests.

mojo/public/c/bindings/tests/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_sdk.gni")
 
 mojo_sdk_source_set("tests") {
   testonly = true
 
   sources = [
     "array_unittest.cc",
     "buffer_unittest.cc",
     "message_unittest.cc",
     "struct_unittest.cc",
+    "testing_util.h",
     "union_unittest.cc",
     "validation_unittest.cc",
   ]
 
   deps = [
     "//testing/gtest",
   ]
 
   mojo_sdk_deps = [
     "mojo/public/cpp/bindings/tests:mojo_public_bindings_test_utils",
     "mojo/public/cpp/bindings/tests:validation_util",
     "mojo/public/cpp/test_support",
     "mojo/public/cpp/system",
     "mojo/public/c/bindings",
 
     # The "_c" suffix'd target compiles the generated C bindings to this mojom
     # target.
     "mojo/public/interfaces/bindings/tests:test_interfaces_c",
   ]
 }