Allow target_arch to select the Mac 64-bit build

build/common.gypi

 # 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 3950 matching lines @@
         ],
       }],
       ['OS=="ios"', {
         'ARCHS': '$(ARCHS_UNIVERSAL_IPHONE_OS)',
         # Just build armv7, until armv7s is correctly tested.
         'VALID_ARCHS': 'armv7 i386',
         'IPHONEOS_DEPLOYMENT_TARGET': '<(ios_deployment_target)',
         # Target both iPhone and iPad.
         'TARGETED_DEVICE_FAMILY': '1,2',
       }],
+      ['target_arch=="x64"', {
+        'ARCHS': [
+          'x86_64'
+         ],
+      }],
     ],
 
     # The Xcode generator will look for an xcode_settings section at the root
     # of each dict and use it to apply settings on a file-wide basis.  Most
     # settings should not be here, they should be in target-specific
     # xcode_settings sections, or better yet, should use non-Xcode-specific
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }