Add BUILD.gn rules to compile third_party/class-dump.

third_party/class-dump/class-dump.gyp

 # Copyright (c) 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.
 
 {
   'variables': {
     'mac_deployment_target': '10.9',
     'mac_sdk_min': '10.9',
   },
   'targets': [
     {
+      # GN version //third_party/class-dump(//build/toolchain/mac:clang_x64)
       'target_name': 'class-dump',
       'toolsets': ['host'],
       'type': 'executable',
       'sources': [
         'src/class-dump.m',
         'src/Source/CDBalanceFormatter.h',
         'src/Source/CDBalanceFormatter.m',
         'src/Source/CDClassDump.h',
         'src/Source/CDClassDump.m',
         'src/Source/CDClassDumpVisitor.h',
@@ skipping 168 matching lines @@
         'src/Source',
       ],
       'xcode_settings': {
         'GCC_PREFIX_HEADER': 'src/class-dump-Prefix.pch',
         'CLANG_ENABLE_OBJC_ARC': 'YES',
         'CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS': 'NO',
       },
     },
   ],  # targets
 }