Should we (probably the CI API just take cbb_workspace_branch in place of cbb_branch when we see it? Aka branch = cbb_workspace_branch ? cbb_workspace_branch : cbb_branch. I assume everything else in the UI stays the same? Do we care about communicating to the user that the root branch was master?

That's correct.

If changing cbb_branch wouldn't directly change behavior on the builder for the initial checkout, I would have updated it instead.

That's easy then :) Thank Don!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/d6a3c091479b10672f297ff2c91eaff33123b507

commit d6a3c091479b10672f297ff2c91eaff33123b507
Author: Don Garrett <dgarrett@google.com>
Date: Thu Aug 23 14:37:16 2018

request_build: Add "cbb_workspace_branch" support.

This change will populate the tag/property cbb_workspace_branch for
builds scheduled by a master builder, or by cros tryjob. The cros
tryjob support is limited to builds that are defined on the master
branch.

This does not affect builds scheduled by LUCI Scheduler.

BUG= chromium:875509 
TEST=run_tests

Change-Id: I61b328ba5a561ec96b56a157f8bdb336f5a8a67e
Reviewed-on: https://chromium-review.googlesource.com/1180478
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Alec Thilenius <athilenius@google.com>

[modify] https://crrev.com/d6a3c091479b10672f297ff2c91eaff33123b507/lib/request_build.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/73148e5035d19d09b4ade4fc32fbd35c85203ff0

commit 73148e5035d19d09b4ade4fc32fbd35c85203ff0
Author: Don Garrett <dgarrett@google.com>
Date: Mon Sep 17 14:19:08 2018

luci_scheduler: Add support for cbb_workspace_branch.

Add support for a new optional build config value "workspace_branch"
that should populate the tag/property cbb_workspace_branch.

This requires adding support for optional tags/properties to
gen_luci_scheduler. The implementation of this has the side effect of
sorting the generated tags/properties, which is reflected in the
unittests.

BUG= chromium:875509 
TEST=run_tests

Change-Id: If9f1706bc9e2e1534088722ea9f59fc7f627494d
Reviewed-on: https://chromium-review.googlesource.com/1180509
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>

[modify] https://crrev.com/73148e5035d19d09b4ade4fc32fbd35c85203ff0/scripts/gen_luci_scheduler.py
[modify] https://crrev.com/73148e5035d19d09b4ade4fc32fbd35c85203ff0/scripts/gen_luci_scheduler_unittest.py
[modify] https://crrev.com/73148e5035d19d09b4ade4fc32fbd35c85203ff0/config/luci-scheduler.cfg