If you add a screenshot of this dialog as it exists now I can list the exact changes that need to be made to Harmonize it.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dcc44ff52e9de6af0034d5b996b8047fab1a972b

commit dcc44ff52e9de6af0034d5b996b8047fab1a972b
Author: Allen Bauer <kylixrd@chromium.org>
Date: Thu Jul 06 18:19:59 2017

Remove close (X) from javascript unload and reload dialogs.

Bug:  652017 
Bug:  652015 
Change-Id: I61970a2755ac95d9b19cf648422dce5585ec427c
Reviewed-on: https://chromium-review.googlesource.com/557941
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Allen Bauer <kylixrd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484675}
[modify] https://crrev.com/dcc44ff52e9de6af0034d5b996b8047fab1a972b/components/app_modal/views/javascript_app_modal_dialog_views.cc
[modify] https://crrev.com/dcc44ff52e9de6af0034d5b996b8047fab1a972b/components/app_modal/views/javascript_app_modal_dialog_views.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dbe28cca6d52f9d2240323948e98bedfeb126c78

commit dbe28cca6d52f9d2240323948e98bedfeb126c78
Author: Allen Bauer <kylixrd@chromium.org>
Date: Thu Jul 06 19:20:08 2017

Use the LayoutProvider to supply the default checkbox/radiobutton borders.

Bug:  652017 
Bug:  652015 
Change-Id: I40188d8827bd32df8eb152861ebf9f422234f803
Reviewed-on: https://chromium-review.googlesource.com/555834
Commit-Queue: Allen Bauer <kylixrd@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484705}
[modify] https://crrev.com/dbe28cca6d52f9d2240323948e98bedfeb126c78/chrome/browser/ui/views/content_setting_bubble_contents.cc
[modify] https://crrev.com/dbe28cca6d52f9d2240323948e98bedfeb126c78/chrome/browser/ui/views/harmony/harmony_layout_provider.cc
[modify] https://crrev.com/dbe28cca6d52f9d2240323948e98bedfeb126c78/ui/views/controls/button/checkbox.cc
[modify] https://crrev.com/dbe28cca6d52f9d2240323948e98bedfeb126c78/ui/views/controls/button/checkbox.h
[modify] https://crrev.com/dbe28cca6d52f9d2240323948e98bedfeb126c78/ui/views/layout/layout_provider.cc
[modify] https://crrev.com/dbe28cca6d52f9d2240323948e98bedfeb126c78/ui/views/layout/layout_provider.h

Load balancing

bouncing to pbos :)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6314cbc42d31aaf3cadd302cfb0ec31dfa7f0578

commit 6314cbc42d31aaf3cadd302cfb0ec31dfa7f0578
Author: Peter Boström <pbos@chromium.org>
Date: Fri Nov 10 00:46:57 2017

Style views::MessageBoxView for Harmony.

Introduces CONTEXT_MESSAGE_BOX_BODY_TEXT and styles it like
CONTEXT_BODY_TEXT_LARGE and STYLE_SECONDARY under Harmony. This
specifically styles the JavaScript dialogs.

Bug:  chromium:652015 ,  chromium:652017 ,  chromium:683346 
Change-Id: I846c33736cfffb368105ea1cd49c2a8079e20b14
Reviewed-on: https://chromium-review.googlesource.com/759409
Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515378}
[modify] https://crrev.com/6314cbc42d31aaf3cadd302cfb0ec31dfa7f0578/chrome/browser/ui/views/harmony/harmony_typography_provider.cc
[modify] https://crrev.com/6314cbc42d31aaf3cadd302cfb0ec31dfa7f0578/ui/views/controls/message_box_view.cc
[modify] https://crrev.com/6314cbc42d31aaf3cadd302cfb0ec31dfa7f0578/ui/views/style/typography.h

The NextAction date has arrived: 2017-11-10

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/977ebe428e23759cdcdb5ea7e342f210604c17f0

commit 977ebe428e23759cdcdb5ea7e342f210604c17f0
Author: Peter Boström <pbos@chromium.org>
Date: Fri Nov 17 03:18:22 2017

Add conditional titlecase to JavaScript dialogs.

Adds titlecase title strings for window.prompt, alert, on-reload
and on-close dialogs.

Bug:  chromium:652015 ,  chromium:652017 ,  chromium:683346 
Change-Id: I197a8ad11d95d0d4f7671bba81f671d8c45206c2
Reviewed-on: https://chromium-review.googlesource.com/773907
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517271}
[modify] https://crrev.com/977ebe428e23759cdcdb5ea7e342f210604c17f0/components/app_modal_strings.grdp

Attaching screenshot @ 150% HDPI on Windows 10. The checkbox only show up on repeat dialog invocations.

Deleted: javascript-onbeforeunload-leave-150hdpi.png 155 KB

	javascript-onbeforeunload-leave-150hdpi.png 155 KB View Download

+ change 'stay' to 'cancel' 
+ remove punctuation on the checkbox string

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/077959a33ff19709c8972d9c0549bdfea605a21a

commit 077959a33ff19709c8972d9c0549bdfea605a21a
Author: Peter Boström <pbos@chromium.org>
Date: Sat Dec 23 05:33:12 2017

Harmony fixes for the JavaScript app-modal dialog.

* Removes punctuation from the prevent-additional-dialogs checkbox.
* Removes the special cancel button titles in favor of a generic cancel
  button.

Bug:  chromium:652015 ,  chromium:652017 
Change-Id: Ib2d997ed7d20b8a1acb10056874b38f1b2480eac
Reviewed-on: https://chromium-review.googlesource.com/843129
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526140}
[modify] https://crrev.com/077959a33ff19709c8972d9c0549bdfea605a21a/chrome/browser/ui/cocoa/javascript_app_modal_dialog_cocoa.mm
[modify] https://crrev.com/077959a33ff19709c8972d9c0549bdfea605a21a/components/app_modal/views/javascript_app_modal_dialog_views.cc
[modify] https://crrev.com/077959a33ff19709c8972d9c0549bdfea605a21a/components/app_modal_strings.grdp