Proposing this as the proper trigger:

     triggered_gitiles=[[
          'https://chromium.googlesource.com/chromium/src',
          ['regexp:refs/tags/[^/]+']
      ]],


I'm currently doing a local sync to see what the tags for release branches look like to ensure that would cover branches, or see if we can be more clever.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/5dbb07d706256e7607fa71f9e2e72784dd8c0e6b

commit 5dbb07d706256e7607fa71f9e2e72784dd8c0e6b
Author: Don Garrett <dgarrett@google.com>
Date: Fri Nov 02 18:57:52 2018

chromeos_config: Use triggers for branched Chrome PFQs.

This attempts to schedule Chrome PFQs on release branches based on
triggers, so that they can be run less frequently, but also pick up
new versions in a more timely fashion.

This is based on discussion from CL:1296625.

BUG= chromium:894970 
TEST=None

Change-Id: I29f95ebc683093e0aee9ce1e122b12bd94fec2d8
Reviewed-on: https://chromium-review.googlesource.com/1306793
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Alec Thilenius <athilenius@google.com>
Reviewed-by: Bernie Thompson <bhthompson@chromium.org>

[modify] https://crrev.com/5dbb07d706256e7607fa71f9e2e72784dd8c0e6b/config/chromeos_config.py
[modify] https://crrev.com/5dbb07d706256e7607fa71f9e2e72784dd8c0e6b/config/luci-scheduler.cfg

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/7f6e7c60e882f80cae6ac8101183f0d168ea51f0

commit 7f6e7c60e882f80cae6ac8101183f0d168ea51f0
Author: Don Garrett <dgarrett@google.com>
Date: Fri Nov 02 21:55:25 2018

chromeos_config: Fix release branch trigger syntax.

CL:1306793 started generating a luci-scheduler.cfg with invalid syntax
in the regular expressions.

BUG= chromium:894970 
TEST=chromeos_config_unittest
     Manually running Chops presubmit against generated file.

Change-Id: Id2c3a38feeef98aded85158287e552aa442bebae
Reviewed-on: https://chromium-review.googlesource.com/1315778
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Bernie Thompson <bhthompson@chromium.org>

[modify] https://crrev.com/7f6e7c60e882f80cae6ac8101183f0d168ea51f0/config/chromeos_config.py
[modify] https://crrev.com/7f6e7c60e882f80cae6ac8101183f0d168ea51f0/config/luci-scheduler.cfg

Believed fixed, but waiting for confirmation in production.