Add testing/iossim/BUILD.gn to build iossim as host for iOS build.

testing/iossim/iossim.gyp

 # Copyright (c) 2012 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.
 
 {
   'variables': {
     'mac_deployment_target': '10.9',
     'mac_sdk_min': '10.9',
     'class_dump_bin': '<(PRODUCT_DIR)/class-dump',
     'class_dump_py': '<(DEPTH)/third_party/class-dump/class-dump.py',
   },
   'conditions': [
     ['OS!="ios" or "<(GENERATOR)"!="xcode" or "<(GENERATOR_FLAVOR)"=="ninja"', {
       'targets': [
         {
+          # GN version //testing/iossim(//build/toolchain/mac:clang_x64)
           'target_name': 'iossim',
           'toolsets': ['host'],
           'type': 'executable',
           'variables': {
             'developer_dir': '<!(xcode-select -print-path)',
             'iphone_sim_path': '<(developer_dir)/../SharedFrameworks',
           },
           'dependencies': [
             '<(DEPTH)/third_party/class-dump/class-dump.gyp:class-dump#host',
           ],
@@ skipping 106 matching lines @@
                 'iossim',
               ],
               'message': 'Generating the iossim executable',
             },
           ],
         },
       ],
     }],
   ],
 }