Noted on the email thread, but perhaps better to copy here for posterity: 
This issue did raise the question of "are different build machines (dev machine chroot, pre-cq, cq) running builds with different compiler flags?" 
If yes, why / can we help ensure that the flags are same everywhere?
The issue happened due to -Werrror on the CQ, and I think the developer was able to build successfully on their machine, so they didn't catch it.  

Re #2:

kirtika@, in this special case https://chromium-review.googlesource.com/c/420974/, what's the flag causing the difference in compiling debugd package? 

Is the arch type (ARM/x86) causing the difference in pre-cqs and cqs ?

yes, this comes down (again) to our pre-existing bug that we don't currently have any x86 bots listed in the pre-cq configs

What's a good candidate? I suppose we can catch this sort of thing with just build_packages and unit test? If so, this is cheap to add (plenty of gce buildslave capacity for this).

x86-alex-no-vmtest-pre-cq i think would be fine

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/7b6bb79ed88eade5c75b4b66670c5419b1aafac4

commit 7b6bb79ed88eade5c75b4b66670c5419b1aafac4
Author: Aviv Keshet <akeshet@chromium.org>
Date: Wed Feb 08 04:14:26 2017

constants: add x86-alex-no-vmtest-pre-cq to default set

BUG= chromium:689224 
TEST=x86-alex-no-vmtest-pre-cq tryjob

Change-Id: I241c65a42cda10321b41a9dce45fb352f9ec8a2c
Reviewed-on: https://chromium-review.googlesource.com/438783
Commit-Ready: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[modify] https://crrev.com/7b6bb79ed88eade5c75b4b66670c5419b1aafac4/lib/constants.py

 Issue 675657  has been merged into this issue.