Fix multiprocess file-locking problem in launch_slaves.py
Fix multiprocess file-locking problem in launch_slaves.py

(SkipBuildbotRuns)
Review URL: https://chromiumcodereview.appspot.com/14205014

git-svn-id: 32fc27f4dcfb6c0385cd9719852b95fe6680452d@8729 2bbb7eff-a529-9590-31e7-b0007b416f81

Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot