This is needed because bot_update changed from getting the cache directory from
the global path module to the gclient config.

BUG=589140

Review URL: https://codereview.chromium.org/1768893003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299143 0039d316-1c4b-4281-b951-d872f2087c98