Unfortunately, the locking mechanism is still flaky on Windows.
bot_update.py will use this, since we can be certain that there
won't be overlapping access to the cache.

R=hinoka@google.com, agable@chromium.org, hinoka@chromium.org
BUG=

Review URL: https://codereview.chromium.org/340953003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@278490 0039d316-1c4b-4281-b951-d872f2087c98