Merge -c 1489236 from trunk to branch-2 to fix MAPREDUCE-5245. Added back constants to JobConf to fix incompatibilities. Contributed by Zhijie Shen.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1489237 13f79535-47bb-0310-9956-ffa450edef68