Add cwd to all shell commands in auto roll scripts.

The v8 root directory is assumed to be the default cwd. All
commands executed in another directory (e.g. the chromium
checkout) need an explicit specification (also in the
tests).

This also fixes several small testing and robustness bugs:
- Get rid of all 'rm ...' shell calls
- Don't leak tmp files/dirs
- Add some forgotten shell calls to the test expectations
- Hardcode the DEPS location (must always be
chromium_dir/DEPS)
- Expect correct return code when terminating gracefully

BUG=chromium:408523
LOG=n
TBR=jarin@chromium.org
TEST=script_test.py

Review URL: https://codereview.chromium.org/540973002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23719 ce2b1a6d-e550-0410-aec6-3dcde31c8c00