WebRTC: Pin SDK version to 10.9 for Mac and iOS.

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_mac_rel.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "config",
       "--spec",
       "cache_dir = None\nsolutions = [{'custom_vars': {'root_dir': 'src'}, 'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'svn://svn-mirror.golo.chromium.org/webrtc/trunk'}, {'custom_vars': {'root_dir': 'src'}, 'deps_file': 'DEPS', 'managed': True, 'name': 'webrtc-limited', 'url': 'svn://svn-mirror.golo.chromium.org/chrome-internal/trunk/webrtc-limited'}]"
     ],
@@ skipping 61 matching lines @@
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
     "env": {
-      "GYP_DEFINES": "clang=1 component=static_library dcheck_always_on=1 fastbuild=1 gomadir='[BUILD]/goma' target_arch=ia32 use_goma=1"
+      "GYP_DEFINES": "clang=1 component=static_library dcheck_always_on=1 fastbuild=1 gomadir='[BUILD]/goma' mac_sdk=10.9 target_arch=ia32 use_goma=1"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/compile.py",
       "--target",
       "Release",
@@ skipping 332 matching lines @@
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"mac_rel\", \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--annotate=gtest",
       "--test-type=voice_engine_unittests",
       "--builder-name=mac_rel",
       "--slave-name=slavename",
       "voice_engine_unittests"
     ],
     "name": "voice_engine_unittests"
   }
 ]