Split the //mojo/public/cpp/test_support:test_utils target.

mojo/public/cpp/test_support/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_sdk.gni")
 
-# GYP version: mojo/public/mojo_public.gyp:mojo_public_test_utils
-mojo_sdk_source_set("test_utils") {
+# This target depends on mojo/public/c/test_support.
+mojo_sdk_source_set("test_support") {
   testonly = true
 
   sources = [
     "lib/test_support.cc",
-    "lib/test_utils.cc",
-    "test_utils.h",
-  ]
-
-  deps = [
-    "//testing/gtest",
+    "test_support.h",
   ]
 
   mojo_sdk_deps = [
     "mojo/public/c/test_support",
     "mojo/public/cpp/system",
   ]
 }
+
+# This target should be "pure Mojo" and have no external dependencies.
+mojo_sdk_source_set("test_utils") {
+  testonly = true
+
+  sources = [
+    "lib/test_utils.cc",
+    "test_utils.h",
+  ]
+
+  mojo_sdk_deps = [ "mojo/public/cpp/system" ]
+}