Add a prototype today extension behind gn flag

ios/build/chrome_build.gni

 # 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/chrome_build.gni")
 import("//build/config/ios/ios_sdk.gni")
 import("//build/config/mac/symbols.gni")
 
 declare_args() {
   # Enable today extension.
   ios_enable_today_extension = true
+  ios_enable_widget_extension = false
 
   # Enable share extension.
   ios_enable_share_extension = true
 
   # Value of the encryption export compliance code. See "Cryptography and
   # U.S. Export Compliance" in "Submitting the App to App Review" in the
   # Apple developer documentation (https://goo.gl/yv1xEF).
   ios_encryption_export_compliance_code = ""
 
   # List of plist templates to merge when generating chrome Info.plist.
   ios_chrome_info_plist_additions = []
 
   # List of plist templates to merge when generating chrome entitlements.
   ios_chrome_entitlements_additions = []
 }
+assert(!(ios_enable_today_extension && ios_enable_widget_extension),
+       "Both today extensions cannot be enabled simultaneously.")
 
 # Configure whether breakpad support is enabled.
 breakpad_enabled = is_official_build && is_chrome_branded
 
 if (breakpad_enabled) {
   breakpad_enabled_as_int = 1
   is_official_release = enable_dsyms && !use_ios_simulator &&
                         current_toolchain == default_toolchain
 } else {
   breakpad_enabled_as_int = 0
@@ skipping 11 matching lines @@
   url_unsecure_scheme = "googlechrome"
   url_x_callback_scheme = "googlechrome-x-callback"
 } else {
   chromium_short_name = "Chromium"
   url_channel_scheme = "chromium-dev"
   url_secure_scheme = "chromiums"
   url_ssoauth_scheme = "$ios_app_bundle_id_prefix.sso.chromium"
   url_unsecure_scheme = "chromium"
   url_x_callback_scheme = "chromium-x-callback"
 }