Add a prototype today extension behind gn flag

ios/chrome/app/BUILD.gn

Index: ios/chrome/app/BUILD.gn
diff --git a/ios/chrome/app/BUILD.gn b/ios/chrome/app/BUILD.gn
index c94edaee7214bbb08849d60e0b9973611c6bfab1..8a47bf8565630b7396d83e472d1a304bc0d5acfd 100644
--- a/ios/chrome/app/BUILD.gn
+++ b/ios/chrome/app/BUILD.gn
@@ -277,6 +277,11 @@ ios_app_bundle("chrome") {
     deps += [ ":today_extension_bundle" ]
   }
 
+  if (!ios_enable_today_extension && ios_enable_proto_today_extension &&

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

I +1-ing olivierrobin earlier comment (use assert to check that only one of the
two extension is enabled), then change this to:

  if (ios_enable_proto_today_extension && current_toolchain ==
default_toolchain) {
    ... 
  }

BTW, I would even recommend refactoring this to the following:

  if (current_toolchain == default_toolchain) {
    if (ios_enable_today_extension) {
      deps += [ ":today_extension_bundle" ]
    }
    if (ios_enable_proto_today_extension) {
      deps += [ ":proto_today_extension_bundle" ]
    }
    if (ios_enable_share_extension) {
      deps += [ ":share_extension_bundle" ]
    }
  }

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

Done.

+      current_toolchain == default_toolchain) {
+    deps += [ ":proto_today_extension_bundle" ]
+  }
+
   if (ios_enable_share_extension && current_toolchain == default_toolchain) {
     deps += [ ":share_extension_bundle" ]
   }
@@ -312,6 +317,21 @@ if (ios_enable_today_extension && current_toolchain == default_toolchain) {
   }
 }
 
+if (!ios_enable_today_extension && ios_enable_proto_today_extension &&

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

ditto, I would recommend refactoring this to

if (current_toolchain == default_toolchain) {
  if (ios_enable_today_extension) {
    bundle_data("today_extension_bundle") {
      ...
    }
  }
  if (ios_enable_proto_today_extension) {
    bundle_data("proto_today_extension_bundle") {
      ..
    }
  }
  ..
}

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

Done.

+    current_toolchain == default_toolchain) {
+  bundle_data("proto_today_extension_bundle") {
+    public_deps = [
+      "//ios/chrome/proto_today_extension",
+    ]
+    sources = [
+      "$root_out_dir/proto_today_extension.appex",
+    ]
+    outputs = [
+      "{{bundle_plugins_dir}}/{{source_file_part}}",
+    ]
+  }
+}
+
 if (ios_enable_share_extension && current_toolchain == default_toolchain) {
   bundle_data("share_extension_bundle") {
     public_deps = [