The first patchset at https://chromium-review.googlesource.com/c/chromium/src/+/1065021 generates the attached dialog (using --enable-features=SecondaryUiMd).  Please take a look and let me know if it's sufficiently harmonious.

... and actually attaching a screenshot...

Deleted: hd_cancelcast_v1.png 18.2 KB

	hd_cancelcast_v1.png 18.2 KB View Download

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3028a826d1da5b9822cdfff31f3d3d7d613e981a

commit 3028a826d1da5b9822cdfff31f3d3d7d613e981a
Author: glevin <glevin@chromium.org>
Date: Thu Jun 07 18:06:55 2018

Harmonize CancelCastingDialog

As per  crbug.com/788014  Comment 1 (and previous dlgs):
- Change button text "OK" -> "Continue"
- Remove X (Close) button from upper right

Bug:  788014 
Test: Cast to a screen while multi-logged in. Try switching profiles.
Try to spot the Harmony.

Change-Id: Icdb122f100d6f0b00129da7534150b7314a17686
Reviewed-on: https://chromium-review.googlesource.com/1065021
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Greg Levin <glevin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565345}
[modify] https://crrev.com/3028a826d1da5b9822cdfff31f3d3d7d613e981a/ash/ash_strings.grd
[modify] https://crrev.com/3028a826d1da5b9822cdfff31f3d3d7d613e981a/ash/system/screen_security/screen_switch_check_controller.cc

This bug requires manual review: There is .grd file changes and we are only 45 days from stable.
Please contact the milestone owner if you have questions.
Owners: cmasso@(Android), kariahda@(iOS), bhthompson@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

*facepalm* I somehow missed that this one had a string change.  The CL is minor, and omrilio@ agrees with me that this can just wait till M-69.  Merge request rescinded.