This is a regression issue, broken in M-68  series, and will soon update other info.
Good Build: 68.0.3433.0 (Revision:559328)
Bad Build: 68.0.3434.0 (Revision:559775)

You are probably looking for a change made after 559737 (known good), but no later than 559738 (first known bad).

CHANGE-LOG URL:

The script might not always return single CL as suspect as some perf builds might get missing due to failure.

https://chromium.googlesource.com/chromium/src/+log/f9535bd6d61d7e0b2cb452e6976a914d4ee62a2b..607e9a387d3b444550e8b107a15b40c62121f424

Suspect: https://chromium.googlesource.com/chromium/src/+/607e9a387d3b444550e8b107a15b40c62121f424

@aee: Could you please check whether this is caused with respect to your change, if not please help us in assigning it to the right owner

Note: Issue is also seen on Mac(10.12.6, 10.13.1, 10.13.5 ) OS

Thank You!

Let's prioritize this fix, since branch cut is coming up pretty soon.

 Issue 845978  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/29bc0a300e670bb159425a47a5ad7bade2cdf386

commit 29bc0a300e670bb159425a47a5ad7bade2cdf386
Author: Esmael El-Moslimany <aee@chromium.org>
Date: Thu May 24 02:46:30 2018

WebUI: ensure padding between title and buttons when body is scrollable

Bug:  844329 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Id9b00c368e32c9cea170af7c6dfaf9a5272e1301
Reviewed-on: https://chromium-review.googlesource.com/1066326
Reviewed-by: Scott Chen <scottchen@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561365}
[modify] https://crrev.com/29bc0a300e670bb159425a47a5ad7bade2cdf386/chrome/browser/resources/print_preview/new/destination_dialog.html
[modify] https://crrev.com/29bc0a300e670bb159425a47a5ad7bade2cdf386/chrome/browser/resources/print_preview/new/destination_list.html
[modify] https://crrev.com/29bc0a300e670bb159425a47a5ad7bade2cdf386/chrome/browser/resources/print_preview/new/search_dialog_css.html
[modify] https://crrev.com/29bc0a300e670bb159425a47a5ad7bade2cdf386/chrome/browser/resources/settings/search_engines_page/search_engine_dialog.html
[modify] https://crrev.com/29bc0a300e670bb159425a47a5ad7bade2cdf386/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.html

Update : 

Retested above issue in latest Canary build # 68.0.3439.0 on Win(7,8,8.1,10), Linux(14.04 LTS) and Mac(10.12.6, 10.13.1, 10.13.5) and the issue is fixed. Now, proper space is seen between separator line and Add/Cancel buttons

Kindly review an attached screen-cast.

Thank you..!

Deleted: Current Behaviour.mp4 231 KB

	Current Behaviour.mp4 231 KB View Download