We're implementing a bootstrap downgrade for potential revert of
https://chromium-review.googlesource.com/c/563036

Currently, the downgrade sees the presence of the landed patch
as a sign to completely reinstall Python. However, this causes
the "python276_bin" directory to be deleted, which both ruins
any running process (notably "service manager" and BuildBot)
and fails due to those running processes, leaving the system in
a broken state.

Instead, we'll view the revert path as a signal to swap in the
old "python.bat" specifically, then fall through to standard
installation detection. Since the upgrade will not actually
delete "python276_bin", the expected revert case will leave it
alone, preserving existing execution environments.

BUG=chromium:740966, chromium:740171
TEST=local
  - Ran simulation of revert path with this change, observed
    better outcome.

Change-Id: I0dfa5924a27bcaba49134272a344f7b9f1d475c5
Reviewed-on: https://chromium-review.googlesource.com/567167Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Daniel Jacques <dnj@chromium.org>