https://uberchromegw.corp.google.com/i/chromeos/builders/master-paladin/builds/16798

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/d41fbd6014b7950e05aaf0fa87536a2128c6d5eb

commit d41fbd6014b7950e05aaf0fa87536a2128c6d5eb
Author: Ningning Xia <nxia@chromium.org>
Date: Thu Nov 02 20:04:06 2017

simple_builders: _HandleChanges when important slaves failed.

When the completion stage only raises ImportantBuilderFailedException,
still run _HandleChanges to submit partial CLs.

BUG= chromium:780875 
TEST=unit_tests

Change-Id: I0cc5895eeeaf7796b3bf1e6ff8a4f3b2eebc69e7
Reviewed-on: https://chromium-review.googlesource.com/751797
Reviewed-by: Ningning Xia <nxia@chromium.org>
Tested-by: Ningning Xia <nxia@chromium.org>

[modify] https://crrev.com/d41fbd6014b7950e05aaf0fa87536a2128c6d5eb/cbuildbot/stages/completion_stages_unittest.py
[modify] https://crrev.com/d41fbd6014b7950e05aaf0fa87536a2128c6d5eb/cbuildbot/stages/generic_stages.py
[modify] https://crrev.com/d41fbd6014b7950e05aaf0fa87536a2128c6d5eb/cbuildbot/builders/simple_builders_unittest.py
[modify] https://crrev.com/d41fbd6014b7950e05aaf0fa87536a2128c6d5eb/cbuildbot/builders/simple_builders.py

Fixed.

https://uberchromegw.corp.google.com/i/chromeos/builders/master-paladin/builds/16801