Windows will sometimes freak out if a file is rewritten while
it's being executed.  That can happen when update_depot_tools.bat
runs.

R=iannucci@chromium.org,maruel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/12755033

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@189429 0039d316-1c4b-4281-b951-d872f2087c98