[GN] Add support for generating Xcode projects.

build/config/ios/rules.gni

Index: build/config/ios/rules.gni
diff --git a/build/config/ios/rules.gni b/build/config/ios/rules.gni
index 4b0592f39db0d889af9e30a180cefcbba20b9992..380cf8e83e9807d5e20a318370375267dd31d304 100644
--- a/build/config/ios/rules.gni
+++ b/build/config/ios/rules.gni
@@ -152,10 +152,12 @@ template("ios_app_bundle") {
       data_deps += [ "//testing/iossim" ]
     }
 
+    product_type = "com.apple.product-type.application"
     bundle_root_dir = "$root_out_dir/$_output_name.app"
     bundle_resources_dir = bundle_root_dir
     bundle_executable_dir = bundle_root_dir
     bundle_plugins_dir = "$bundle_root_dir/Plugins"
+    assert(product_type != "", "force use of product_type")

[brettw, 2016/04/28 21:16:17]

I don't understand this line. You just set it 4 lines above.

[sdefresne, 2016/04/30 11:25:02]

Oh, this is because the current patchset introduce the "product_type" property
to "create_bundle" targets. So the bot complain about unused variable
"product_type" as they run "gn gen". This force an use of the variable so that
the bot do not complain. An alternative is to move the "product_type" assignment
to a separate CL.

   }
 
   # TODO(crbug.com/297668):