Fix the default revision number source for GetZipFileNames.
Fix the default revision number source for GetZipFileNames.

slave_utils.py gets the build number from build_dir, which in ninja-mode, is
'src/out' which doesn't exist on the tester bots. This results in a false revison
of 0.



Review URL: https://chromiumcodereview.appspot.com/11419190

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@169791 0039d316-1c4b-4281-b951-d872f2087c98

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