WebRTC: Update Macs to 10.10 (with Xcode 7)

masters/master.tryserver.webrtc/master.cfg

 #!/usr/bin/env python
 #  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
 #
 #  Use of this source code is governed by a BSD-style license
 #  that can be found in the LICENSE file in the root of the source
 #  tree. An additional intellectual property rights grant can be found
 #  in the file PATENTS.  All contributing project authors may
 #  be found in the AUTHORS file in the root of the source tree.
 
 import os
@@ skipping 50 matching lines @@
   {'name': 'mac_compile_x64_dbg', 'slavebuilddir': 'mac64'},
   {'name': 'mac_compile_x64_rel', 'slavebuilddir': 'mac64'},
   {'name': 'mac_dbg', 'slavebuilddir': 'mac32'},
   {'name': 'mac_rel', 'slavebuilddir': 'mac32'},
   {'name': 'mac_x64_dbg', 'slavebuilddir': 'mac64'},
   {'name': 'mac_x64_rel', 'slavebuilddir': 'mac64'},
   {'name': 'mac_x64_gn_dbg', 'slavebuilddir': 'mac_gn'},
   {'name': 'mac_x64_gn_rel', 'slavebuilddir': 'mac_gn'},
   {'name': 'mac_baremetal', 'slavebuilddir': 'mac32'},
   {'name': 'mac_swarming', 'slavebuilddir': 'mac64_swarming'},
-  {'name': 'mac_rel_xcode7', 'slavebuilddir': 'mac64'},
   {'name': 'ios_dbg', 'slavebuilddir': 'mac32'},
   {'name': 'ios_rel', 'slavebuilddir': 'mac32'},
   {'name': 'ios_arm64_dbg', 'slavebuilddir': 'mac64'},
   {'name': 'ios_arm64_rel', 'slavebuilddir': 'mac64'},
   {'name': 'ios32_sim_dbg', 'slavebuilddir': 'mac32'},
   {'name': 'ios64_sim_dbg', 'slavebuilddir': 'mac64'},
   {'name': 'ios64_gn_dbg', 'slavebuilddir': 'mac64_gn'},
   {'name': 'ios64_gn_rel', 'slavebuilddir': 'mac64_gn'},
   {'name': 'mac_asan', 'slavebuilddir': 'mac_asan'},
   {'name': 'linux_compile_dbg', 'slavebuilddir': 'linux64'},
@@ skipping 134 matching lines @@
 # Must be at least 2x the number of slaves.
 c['eventHorizon'] = 100
 # Must be at least 2x the number of on-going builds.
 c['buildCacheSize'] = 100
 
 ####### PROJECT IDENTITY
 
 # The 'projectURL' string will be used to provide a link
 # from buildbot HTML pages to your project's home page.
 c['projectURL'] = 'http://dev.chromium.org/developers/testing/try-server-usage'