This reverts commit 873c28d1.

Reason for revert: Broken on Windows (crbug.com/762389)

Original change's description:
> Capture ctrl-c in presubmit multiprocessing pool
> 
> Presubmit spins up lots of multiprocessing processes to run
> each individual test. If you cancel your presubmit run with
> <ctrl>+c, that signal gets passed through to each of those,
> which then raises its own KeyboardInterrupt, and prints its
> own stacktrace.
> 
> This change has each member of the multiprocessing pool instead
> exit gracefully (albeit with an error code) so that only the
> parent process prints its stacktrace.
> 
> R=​michaelpg@chromium.org
> 
> Bug: 635196
> Change-Id: If9081910a359889a43bc1b72c91a859ebe37a1d6
> Reviewed-on: https://chromium-review.googlesource.com/651764
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> Commit-Queue: Aaron Gable <agable@chromium.org>

TBR=iannucci@chromium.org,agable@chromium.org,michaelpg@chromium.org

Change-Id: Ib8e5b2f59b0060dfbfbeba348e211db292318b3b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 635196
Reviewed-on: https://chromium-review.googlesource.com/653434Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>