Fix exception introduced in r197711 for non-swarm build masters.
Fix exception introduced in r197711 for non-swarm build masters.

getProperty() will throw if the property is not present.

TBR=csharp@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/14849010

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@197899 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build