Add a prototype today extension behind gn flag

ios/build/chrome_build.gni

Index: ios/build/chrome_build.gni
diff --git a/ios/build/chrome_build.gni b/ios/build/chrome_build.gni
index a5a1394822c8dfaa8b7c65f6034326cde8fe6ff0..db63437ad89215871f9c6da799c3eeb055b8a3c4 100644
--- a/ios/build/chrome_build.gni
+++ b/ios/build/chrome_build.gni
@@ -9,6 +9,7 @@ import("//build/config/mac/symbols.gni")
 declare_args() {
   # Enable today extension.
   ios_enable_today_extension = true
+  ios_enable_proto_today_extension = false
 
   # Enable share extension.
   ios_enable_share_extension = true
@@ -24,6 +25,8 @@ declare_args() {
   # List of plist templates to merge when generating chrome entitlements.
   ios_chrome_entitlements_additions = []
 }
+assert(!(ios_enable_today_extension && ios_enable_proto_today_extension),
+       "Both today extensions cannot be enabled simultaneously.")
 
 # Configure whether breakpad support is enabled.
 breakpad_enabled = is_official_build && is_chrome_branded