Migrate CIPD bundle installations to stable and remove support in
Windows bootstrap code for legacy (non-Windows) bootstrap installations.

This includes removal of support for
- ZIP installation paths - everything through CIPD now.
- Python legacy code (though cleanup logic still exists).
- JavaScript downloads.
- Separate Git package versioning.

This is a push from "bleeding edge" to production.

BUG=chromium:740171
TEST=local, prod-exemplar
  - Tested cold and warm upgrade and downgrade.
  - Tested upgrade and downgrade step via "update_scripts".

Change-Id: I3dc4392ef2eaa2d7cf829d560128b74e4dbd2cba
Reviewed-on: https://chromium-review.googlesource.com/567767
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>