Add a prototype today extension behind gn flag

ios/BUILD.gn

Index: ios/BUILD.gn
diff --git a/ios/BUILD.gn b/ios/BUILD.gn
index a3f2f48dfae9312dfdb4a8d61404c584a027f4fd..593d212b5c780717d57f1ceace124767bcea4211 100644
--- a/ios/BUILD.gn
+++ b/ios/BUILD.gn
@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//ios/build/chrome_build.gni")
+
 assert(!is_component_build, "component build is unsupported on iOS")
 
 declare_args() {
@@ -41,10 +43,15 @@ group("all") {
       "//ios/web:all_tests",
       "//ios/web/shell/test:all_tests",
 
-      # Those dependencies are currently only used from downstream code
-      # and will be removed once the dependent code has been upstreamed.
+      # today_extension and share_extension dependencies are currently
+      # only used from downstream code and will be removed once the
+      # dependent code has been upstreamed.

[Olivier, 2017/01/17 10:13:05]

I think we can remove this.
Can you see with sdefresne?

[sdefresne, 2017/01/17 10:39:07]

Yes, just revert the whole file. I'll remove the dependency in a separate CL.

[lody, 2017/01/17 17:31:45]

Done.

       "//ios/chrome/share_extension",
-      "//ios/chrome/today_extension",
     ]
+    if (!ios_enable_today_extension && ios_enable_proto_today_extension) {

[Olivier, 2017/01/17 10:13:05]

This will add dep to today_extension if both flags are false.

[lody, 2017/01/17 17:31:45]

Acknowledged.

+      deps += [ "//ios/chrome/proto_today_extension" ]
+    } else {
+      deps += [ "//ios/chrome/today_extension" ]
+    }
   }
 }