The upcoming CL, https://chromium-review.googlesource.com/c/563036,
updates "bootstrap/win"'s mechanism. However, if that patch needs to be
reverted, its "python.bat" stub may still exist in the reverted
repository, and will continue to point to the reverted toolchain.

We don't have a good way to detect whether we should revert "python.bat".
Instead, we will look a file generated by the CL,
"//python_bin_reldir.txt". If we find it, we will assume that the updated
"python.bat" is in use, and reinstall.

Note that if that CL lands and does not require a revert, this logic
will be deleted. It's just there as a safety mechanism to allow a safe
revert.

BUG=chromium:740171
TEST=local

Change-Id: Ifc638cf0512d2a889c37fbf6b8e3f7a3269331b1
Reviewed-on: https://chromium-review.googlesource.com/566073
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>