bug#32882: 27.0.50; thin border around emacs frame on macOS mojave

[Aaron Jensen]

emacs -Q
M-x load-theme manoj-dark

See
https://dzwonsemrish7.cloudfront.net/items/272S1013403W1e1i2W1Y/Image%202018-09-29%20at%2011.44.39%20PM.png

vs

https://cl.ly/115ddf656a41/Image%202018-09-30%20at%2012.04.17%20AM.png

Since upgrading to Mojave there is a thin border around the frame that
was not there prior to Mojave if my memory serves. The border's color
appears to be influenced by the theme, but I don't know how. Is there an
option for this I don't know about or is this some unintentional change
triggered by Mojave?

In GNU Emacs 27.0.50 (build 1, x86_64-apple-darwin18.0.0, NS appkit-1671.00 
Version 10.14 (Build 18A391))
 of 2018-09-29 built on aaron-mbt.local
Repository revision: 6217746dd64b43a2a2b3b66ab50cfbbfc984f36c
Windowing system distributor 'Apple', version 10.3.1671
System Description:  Mac OS X 10.14