Configure WebKit Mac10.9 to be able to build via MB.

tools/mb/mb_config.pyl

 # 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.
 
 {
   # This is a map of buildbot master names -> buildbot builder names ->
   # config names (where each config name is a key in the 'configs' dict,
   # below). MB uses this dict to look up which config to use for a given bot.
   'masters': {
     # Take care when changing any of these builders to ensure that you do not
@@ skipping 492 matching lines @@
       'WebKit Linux Leak': 'swarming_gn_release_bot_x64',
       'WebKit Linux MSAN': 'swarming_gn_msan_release_bot_x64',
       'WebKit Linux Trusty': 'swarming_gn_release_bot_x64',
       'WebKit Linux': 'swarming_gn_release_bot_x64',
       'WebKit Mac Builder (dbg)': 'swarming_gyp_debug_bot',
       'WebKit Mac Builder': 'swarming_gyp_release_bot_x64',
       'WebKit Mac10.10': 'none',
       'WebKit Mac10.11 (dbg)': 'none',
       'WebKit Mac10.11 (retina)': 'swarming_gyp_release_bot_x64',
       'WebKit Mac10.11': 'none',
-      'WebKit Mac10.9': 'none',
+      'WebKit Mac10.9': 'swarming_gyp_release_bot_x64',
       'WebKit Win Builder (dbg)': 'swarming_gyp_debug_bot_minimal_symbols_x86',
       'WebKit Win Builder': 'swarming_gyp_release_bot_minimal_symbols_x86',
       'WebKit Win x64 Builder (dbg)':
         'swarming_gyp_debug_bot_minimal_symbols_x64',
       'WebKit Win x64 Builder': 'swarming_gyp_release_bot_minimal_symbols_x64',
       'WebKit Win10': 'none',
       'WebKit Win7 (dbg)': 'none',
       'WebKit Win7': 'none',
     },
 
@@ skipping 1334 matching lines @@
       'gn_args': 'is_component_build=true enable_iterator_debugging=false',
       'gyp_defines': 'component=shared_library build_for_tool=drmemory',
     },
 
     'valgrind': {
       # TODO: add gn_args for 'build_for_tool=memcheck'
       'gyp_defines': 'build_for_tool=memcheck',
     }
   },
 }