Sometimes, removing lockfiles fails on windows, for obscure reasons.
Following advice from gclient_utils.rmtree, lock file removal has
been improved by using the builtin executable 'del' and retrying 3
times in case it fails.

BUG=

Review URL: https://codereview.chromium.org/240653002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@264557 0039d316-1c4b-4281-b951-d872f2087c98