Set the gyp DEPTH variable as relative path.

After r18439, it's an absolute variable which confuses gy

LOG=n
R=ulan@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/119293004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18440 ce2b1a6d-e550-0410-aec6-3dcde31c8c00