Using "set /p" to read a file fails when STDOUT is not connected.
Replace it with a method of reading using "for".

In the Python batch file, we eat up some of our sled buffer in order to
accommodate currently-running Python scripts. They should now resume
past the end of the file without issue. The sled was oversized, so this
shouldn't affect upgrade paths.

BUG=chromium:746602
TEST=local
  - On Windows machine, tested upgrade from pre-rework and current
    bootstraps. This code works on both, and no running Python
    processes were disrupted on exit, suggesting the sled change is
    sound.
R=iannucci@chromium.org, smut@chromium.org

Change-Id: Ica51999d672dd4e448fdad797bffc06ec9e9febf
Reviewed-on: https://chromium-review.googlesource.com/578725
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: smut <smut@google.com>