Disable warning 4702 which affects map, xlist and others on vs2012 and vs2013.

BUG=3584
TESTED=python webrtc\build\gyp_webrtc -G msvs_version=2013 & ninja -C out\Release
R=pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/13089004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6801 4adac7df-926f-26a2-2b94-8c16560cd09d

talk/build/common.gypi

@@ -110,6 +110,12 @@
         'defines': [
           'WEBRTC_WIN',
         ],
+        'msvs_disabled_warnings': [
+          # https://code.google.com/p/chromium/issues/detail?id=372451#c20
+          # Warning 4702 ("Unreachable code") should be re-enabled once
+          # users are updated to VS2013 Update 2.
+            4702,
+        ],
       }],
       ['OS=="ios"', {
         'defines': [