Make Lockfile._remove_lockfile more robust on win32
Make Lockfile._remove_lockfile more robust on win32

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: bd64dd6fa6f3f0ed0c0666d1018379882b742947@264557 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools