[ios] Adds test hooks to enable/disable functionality while testing.

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 = [
     "deferred_initialization_runner.h",
     "deferred_initialization_runner.mm",
     "safe_mode_crashing_modules_config.h",
     "safe_mode_crashing_modules_config.mm",
@@ skipping 14 matching lines @@
     "deferred_initialization_runner_unittest.mm",
     "safe_mode_util_unittest.cc",
   ]
   deps = [
     ":app",
     "//base",
     "//base/test:test_support",
     "//testing/gtest",
   ]
 }
+
+source_set("tests_hook") {
+  # This target is intentionally not test-only, as the main app depends on it.
+  sources = [
+    "tests_hook.h",
+  ]
+}
+
+source_set("tests_fake_hook") {
+  # This target is intentionally not test-only, as the main app depends on it.
+  sources = [
+    "tests_fake_hook.mm",
+  ]
+  deps = [
+    ":tests_hook",
+  ]
+}