Prepare chrome_tests.py for relative build dirs.

No intended behavior change.

BUG=294387
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/31283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229717 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229717