Failure is:

Traceback (most recent call last):
  File "C:\b\rr\tmp5o16op\rw\checkout\scripts\slave\chromium\archive_build.py", line 663, in <module>
    sys.exit(main())
  File "C:\b\rr\tmp5o16op\rw\checkout\scripts\slave\chromium\archive_build.py", line 659, in main
    return s.ArchiveBuild()
  File "C:\b\rr\tmp5o16op\rw\checkout\scripts\slave\chromium\archive_build.py", line 424, in ArchiveBuild
    self.options.arch)
  File "C:\b\rr\tmp5o16op\rw\checkout\scripts\common\archive_utils.py", line 80, in __init__
    self._files_cfg = self._ParseFilesCfg(files_file)
  File "C:\b\rr\tmp5o16op\rw\checkout\scripts\common\archive_utils.py", line 116, in _ParseFilesCfg
    execfile(files_file, exec_globals)
  File "C:\b\c\b\win\src\chrome\tools\build\win\FILES.cfg", line 807
    'buildtype': ['official'],

Obvious culprit r555244.

"""
  # ChromeDriver binary:
  {
    'filename': 'chromedriver.exe',
    'arch': ['32bit']     <--- MISSING A COMMA
    'buildtype': ['official'],
    'archive': 'chromedriver_win32.zip',
    'optional': ['official'],
  },
"""

I will just insert a comma since this is an obvious fix. Since it's build config, I'm not sure if landing a fix is sufficient or whether something needs to be pushed out.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/40dd36700d20278f4872bf3ec2b31a05f2a8eb97

commit 40dd36700d20278f4872bf3ec2b31a05f2a8eb97
Author: Matt Giuca <mgiuca@chromium.org>
Date: Wed May 02 07:17:03 2018

Fix syntax error in FILES.cfg (build config).

Introduced in r555244.

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 835950,  838793 
Change-Id: I32f4aaf40b36884a5f9972d8921b76a070515967
Tbr: crouleau@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1039210
Reviewed-by: Matt Giuca <mgiuca@chromium.org>
Commit-Queue: Matt Giuca <mgiuca@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555323}
[modify] https://crrev.com/40dd36700d20278f4872bf3ec2b31a05f2a8eb97/chrome/tools/build/win/FILES.cfg

 Issue 838792  has been merged into this issue.

This should be fixed now. Will keep an eye on the bots.