- Update "gsutil.py" to be cooperatively safe when invoked
  multiple times simultaneously.
- Allow the cache directory to be overridden by the
  DEPOT_TOOLS_GSUTIL_BIN_DIR environment variable.
- Add a "--clean" flag to force "gsutil.py" to do a clean download.

BUG=chromium:452497
TEST=local
  - for i in `seq 1 50`; do ./gsutil.py --clean -- version&; done

Review URL: https://codereview.chromium.org/1346213003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296772 0039d316-1c4b-4281-b951-d872f2087c98