Fix relative path on slaves that breaks cygwin job paths.
Fix relative path on slaves that breaks cygwin job paths.

BUG=384012
R=hinoka@google.com
NOTRY=true

Review URL: https://codereview.chromium.org/335483005

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@276812 0039d316-1c4b-4281-b951-d872f2087c98

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