This is happening on https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win%20Builder as well as https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win%20Builder%20%28dbg%29

Dan, yesterday I overheard you mentioning some issues about windows bootstrap -- this appears to be a case where there is a problem getting set up for a build; maybe git is not available. Do you have any pointers about what to look at next?

This builder is using a really old CIPD client. That flag has been rolled out for weeks.

Fix is: https://chromium-review.googlesource.com/c/570682/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/depot_tools/+/68f6f279b4f8550624354e0bf795ec60c2befc22

commit 68f6f279b4f8550624354e0bf795ec60c2befc22
Author: Dan Jacques <dnj@chromium.org>
Date: Thu Jul 13 18:37:31 2017

[bootstrap/win] Always use "cipd.bat".

CIPD flags change, and using "cipd.exe" directly skips the selfupdate
step. Remove the optimization and always call "cipd.bat".

BUG= chromium:742448 
TEST=None
R=iannucci@chromium.org, sergeyberezin@chromium.org

Change-Id: Ieb353c695a8ef58a51d046f102266d4926a3447c
Reviewed-on: https://chromium-review.googlesource.com/570682
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Sergey Berezin <sergeyberezin@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

[modify] https://crrev.com/68f6f279b4f8550624354e0bf795ec60c2befc22/bootstrap/win/win_tools.bat

Builds that haven't encountered this should be fixed now; however, builds that already hit this issue will need to be manually fixed. Fix is simple: go into "depot_tools" and run "cipd.bat" to self-update.

$ cd /cygdrive/e/b/depot_tools
$ cipd.bat

Hopefully we caught this fast enough that it didn't spread too far.