Add support for building "ios_chrome_unittests" with gn.

ios/public/test/BUILD.gn

+# Copyright 2015 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.
+
+source_set("test") {
+  testonly = true
+  sources = [
+    "fake_string_provider.cc",
+    "fake_string_provider.h",
+    "fake_sync_service_factory.cc",
+    "fake_sync_service_factory.h",
+    "test_chrome_browser_provider.h",
+    "test_chrome_browser_provider.mm",
+    "test_chrome_provider_initializer.cc",
+    "test_chrome_provider_initializer.h",
+    "test_keyed_service_provider.cc",
+    "test_keyed_service_provider.h",
+    "test_updatable_resource_provider.h",
+    "test_updatable_resource_provider.mm",
+  ]
+
+  deps = [
+    "//base",
+    "//components/keyed_service/core",
+    "//components/keyed_service/ios",
+    "//ios/public/provider/chrome/browser",
+    "//sync",
+    "//testing/gtest",
+  ]
+}