[Cronet] Add BUILD.gn for legacy ios/CrNet project.

ios/crnet/test/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("//build/config/ios/rules.gni")
+import("//testing/test.gni")
+
+test("crnet_test") {
+  testonly = true
+  sources = [
+    "crnet_http_tests.mm",
+    "crnet_test_runner.mm",
+  ]
+
+  deps = [
+    "//base",
+    "//base:i18n",
+    "//ios/crnet:crnet_sources",
+    "//ios/third_party/gcdwebserver",
+    "//net:test_support",
+    "//third_party/icu",
+  ]
+}