Make MIME boundary harder to accidentally match.
This is just making the same change to the copy of upload.py that is in depot_tools.

BUG=419156
R=iannucci@chromium.org

Original Review URL: https://codereview.chromium.org/624513002

Review URL: https://codereview.chromium.org/639503002

git-svn-id: http://src.chromium.org/chrome/trunk/tools/depot_tools@292432 4ff67af0-8c30-449e-8e8b-ad334ec8d88c