[GN] Add support for generating Xcode projects.

BUILD.gn

 # Copyright (c) 2013 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.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 207 matching lines @@
       "//ios/chrome/browser",
       "//ios/chrome/common",
       "//ios/net:ios_net_unittests",
       "//ios/public/provider/chrome/browser",
       "//ios/public/provider/web",
       "//ios/testing:ocmock_support_unittest",
       "//ios/third_party/earl_grey",
       "//ios/web:ios_web_inttests",
       "//ios/web:ios_web_unittests",
       "//ios/web/shell:ios_web_shell",
+      "//tools/gn(${host_toolchain})",

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

This is for convenience when working with iOS and on gn at the same time.

[brettw, 2016/04/27 23:09:30]

I think this makes it a bit confusing about what these lists of targets are for.
I usually find it easiest to build GN itself into a different output directory
than you're targeting anyway.

[sdefresne, 2016/04/28 09:29:12]

Ack, removed.

     ]
   }
 
   deps += root_extra_deps
 
   if (enable_extensions) {
     deps += [ "//extensions/shell:app_shell_unittests" ]
   }
 
   if (enable_media_router) {
@@ skipping 690 matching lines @@
   assert(target_name != "")  # Mark as used.
   sources = invoker.actual_sources
   assert(
       sources == invoker.actual_sources,
       "Do not use a platform name in your output directory (found \"$root_build_dir\"). http://crbug.com/548283")
 }
 
 assert_valid_out_dir("_unused") {
   actual_sources = [ "$root_build_dir/foo" ]
 }