[GN] Add support for generating Xcode projects.

build/config/ios/rules.gni

 # Copyright 2015 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/ios/ios_sdk.gni")
 import("//build/config/mac/rules.gni")
 
 template("ios_info_plist") {
   info_plist(target_name) {
     extra_substitutions = []
@@ skipping 134 matching lines @@
       ":$_bundle_data_info_plist",
     ]
 
     if (use_ios_simulator) {
       if (!defined(data_deps)) {
         data_deps = []
       }
       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")

[sdefresne, 2016/04/27 17:41:53]

I'll either remove this in a later CL or split my CL in two depending on what
you prefer.

   }
 
   # TODO(crbug.com/297668):
   # - add support for codesigning,
   # - find a way to make "ninja -C out/Default base_unittests.app" work as
   #   an alias to "ninja -C out/Default base_unittests" (for convenience
   #   and compatibility with gyp),
 }
 
 # Compile a xib or storyboard file and add it to a bundle_data so that it is
@@ skipping 235 matching lines @@
                              [
                                "testonly",
                                "visibility",
                              ])
       public_deps = [
         ":$_framework_target+link",
       ]
     }
   }
 }