WebRTC: Update Macs to 10.10 (with Xcode 7)

masters/master.tryserver.webrtc/slaves.cfg

Index: masters/master.tryserver.webrtc/slaves.cfg
diff --git a/masters/master.tryserver.webrtc/slaves.cfg b/masters/master.tryserver.webrtc/slaves.cfg
index d6e5c7c8f9fc85c2c437fc19805e65a4c4e7f921..74f30e92f6801e8b7758ac84104a1af9406cd663 100644
--- a/masters/master.tryserver.webrtc/slaves.cfg
+++ b/masters/master.tryserver.webrtc/slaves.cfg
@@ -95,7 +95,7 @@ slaves = [
   {
     'master': 'WebRTCTryServer',
     'os': 'mac',
-    'version': '10.8',
+    'version': '10.10',
     'bits': '64',
     'builder': [
       'mac_asan',
@@ -121,23 +121,13 @@ slaves = [
     ],
     'hostname': 'vm%d-m4' % i,
   } for i in range(510, 513) + range(665, 667) + range(738, 741) +
-             range(818, 820) + [748]
+             range(818, 820) + [748, 1033]
 ] + [
-  {
-    'master': 'WebRTCTryServer',
-    'os': 'mac',
-    'version': '10.10',
-    'bits': '64',
-    'builder': [
-      'mac_rel_xcode7',
-    ],
-    'hostname': 'vm1033-m4',
-  },
   # Mac bare-metal machines with audio devices and headsets.
   {
     'master': 'WebRTCTryServer',
     'os': 'mac',
-    'version': '10.8',
+    'version': '10.10',
     'bits': '64',
     'builder': [
       'mac_baremetal',