Add support for building "ios_chrome_unittests" with gn.

ios/chrome/app/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") {
+  sources = [
+    "UIApplication+ExitsOnSuspend.h",
+    "UIApplication+ExitsOnSuspend.mm",
+    "deferred_initialization_runner.h",
+    "deferred_initialization_runner.mm",
+    "safe_mode_crashing_modules_config.h",
+    "safe_mode_crashing_modules_config.mm",
+    "safe_mode_util.cc",
+    "safe_mode_util.h",
+  ]
+
+  deps = [
+    "//base",
+    "//ios/chrome/browser",
+  ]
+
+  libs = [
+    "Foundation.framework",
+    "UIKit.framework",
+  ]
+}