[content shell] reset the CWD after each layout test

BUG=111316
R=marja@chromium.org

Review URL: https://codereview.chromium.org/11633017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@173906 0039d316-1c4b-4281-b951-d872f2087c98