Filed b/37896339. Nothing has changed on our end.

 Issue 717009  has been merged into this issue.

Workaround:

1/ Find the index of your chromium.org account in the gaia cookie:

  a/ open a new tab
  b/ Navigate to https://accounts.google.com/accountchooser
  c/ Find the chromium.org account in the list
  d/ Indices are zero-based, so top account is index 0, second account is 1, etc.

2/ Clear all cookies on https://codereview.chromium.org/

  a/ open a new tab
  b/ Navigate to https://codereview.chromium.org/
  c/ click on the pad-lock to the left of the URL address bar
  d/ click the "X in use" link under "Cookies"
  e/ clear all the cookies shown
  f/ click close

3/ Force login with specific account

  a/ refresh https://codereview.chromium.org/ tab
  b/ You should now see the login link at the top right of the page
  c/ Right click the login link and choose "Copy link address"
  d/ Paste the link into address bar and append "&authuser=X", where X is the
     index determined in step 2 above
  e/ Press enter to navigate

If you are not signed in to codereview with your chromium account, first make sure the account is actually valid.  See if you can for example read email of the chromium.org account.  If you can't, sign in as required.  Then repeat all the steps above making sure to pick the correct index.

Issue 720197 has been merged into this issue.

This is really annoying, but thank you for the workaround - I figured there must be some URL param I could use.

After you've cleared cookies, assuming your @chromium account is the second user, this URL ought to do it:
https://www.google.com/accounts/ServiceLogin?service=ah&passive=true&continue=https://appengine.google.com/_ah/conflogin%3Fcontinue%3Dhttps://codereview.chromium.org/&ltmpl=gm&authuser=1

Gerrit is now the preferred code review system (https://polygerrit.appspot.com) over Rietveld. Please see the aforementioned link for more info.

> Gerrit is now the preferred code review system over Rietveld.

That's not true: I know we are transitioning there but it's not done yet. The last email about PolyGerrit was "ready for general testing". We are testing it, but:

1. There are still issues with the new system.
2. There are heaps of ongoing reviews in Reitveld.
3. As a reviewer, I have no choice of which code review tool to use (it's up to the author).

So Reitveld is still very much being used for Chromium code reviews. I don't expect any more feature work or old bug fixes, but this is a new regression that makes Reitveld almost unusable in a lot of situations.

We fixed this with the latest deployment:
https://chromium.googlesource.com/infra/infra/+/12ed3aa579a23657ee29ccbaf02f351f49684cd0

AppEngine recently deployed a change which caused the sign-in process to skip the account picker entirely if you only had cookies present for one Google account. It would just assume that that's the account you wanted to use. The change above forces AppEngine to present you with the account picker no matter what, so that you can choose your google/chromium account as appropriate.

I just verified that I can no longer reproduce this bug, starting with a clean slate, starting with a single account signed in, and starting with multiple accounts signed in.

#10 Excellent, thanks! I have verified that it's fixed.