Add support for building "ios_chrome_unittests" with gn.

ios/chrome/common/app_group/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("app_group") {
+  sources = [
+    "app_group_constants.h",
+    "app_group_constants.mm",
+    "app_group_metrics.h",
+    "app_group_metrics.mm",
+  ]
+
+  deps = [
+    "//base",
+    "//components/version_info",
+  ]
+
+  libs = [
+    "CoreGraphics.framework",
+    "Foundation.framework",
+  ]
+}
+
+# This target will be included into application extensions and the list
+# of its dependencies must be kept as short as possible.
+source_set("client") {
+  sources = [
+    "app_group_metrics_client.h",
+    "app_group_metrics_client.mm",
+  ]
+
+  deps = [
+    ":app_group",
+  ]
+}
+
+source_set("main_app") {
+  sources = [
+    "app_group_metrics_mainapp.h",
+    "app_group_metrics_mainapp.mm",
+  ]
+
+  deps = [
+    ":app_group",
+  ]
+}