It looks like they are layer backed at the moment.

I stand corrected, see  issue 863264 .

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f9f97f021be825e8470317d60b7e664fa391932d

commit f9f97f021be825e8470317d60b7e664fa391932d
Author: Sidney San Martín <sdy@chromium.org>
Date: Fri Jul 13 20:21:07 2018

Fix missing rounded corners on MacViews app windows.

Two changes:

- Make all Views windows layer backed (they mostly were already).

- Use _titlebarHeight to hide the titlebar for app windows. _hidingTitlebar
  resulted in the top corners not being clipped (as if the window were a
  sheet).

Bug:  863264 ,  862672 
Change-Id: I9647afdcba464cc17b88cc2d752bf273a0f6ab89
Reviewed-on: https://chromium-review.googlesource.com/1136520
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575047}
[modify] https://crrev.com/f9f97f021be825e8470317d60b7e664fa391932d/chrome/browser/ui/views/frame/native_widget_mac_frameless_nswindow.mm
[modify] https://crrev.com/f9f97f021be825e8470317d60b7e664fa391932d/ui/views/cocoa/bridged_native_widget.mm