Able to reproduce the issue on MAc 10.12.5 using chrome #61.0.3134.0 as per the above steps:
Manual Bisect info:
------------------
Good build:61.0.3132.0-Revision-479900
Bad build:61.0.3134.0-Revision-480301

Per revision bisect info:
------------------------
You are probably looking for a change made after 480274 (known good), but no later than 480275 (first known bad).
CHANGELOG 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/917d881740c08a34d7a83ec324fe3c4854f19de0..eba855f22deff3a00b76384582808925841f4be5

Possible suspect:
---------------
https://chromium.googlesource.com/chromium/src/+/eba855f22deff3a00b76384582808925841f4be5

Estade@Could you please take a look & reassign to the right owner if it is not related to your change.

Adding 'Release block stable' label as it is regressed recently.Please remove if it is not the case.
Thanks..!!

So it appears that unlike Views, Cococa draws the bottom stroke opaquely (as opposed to on top of the bg color, honoring alpha). The code is here[1]. +tapted, do you know how to fix this or who else might have the cycles to help? Seems like it should be roughly a one liner.

[1] https://cs.chromium.org/chromium/src/chrome/browser/ui/cocoa/bookmarks/bookmark_bar_toolbar_view.mm?rcl=5ea087fcae356e2f872d63d4aed4aecf20169135&l=82

It should be compositing on the background..

My guess is that the line

    strokeColor = [strokeColor colorWithAlphaComponent:0.5];

is replacing the 0x20 alpha with 0x80 alpha rather than multiplying it to be 0x10 alpha.

The fix is probably just to delete that line... I'll take a peep.

Deleting seems to work. Attached is left-to-right: dev, canary, fixed@HEAD

With the fix, the color is a *tiny* bit lighter. Certainly in incognito. But I think that was the intent of r480275

https://codereview.chromium.org/2946183002

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	Screen Shot 2017-06-21 at 11.10.36 am.png 19.6 KB View Download

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	Screen Shot 2017-06-21 at 11.12.42 am.png 20.1 KB View Download

that wasn't technically the intent of that change, but I think this brings Cocoa into better alignment with Views.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/36d91d332d9c8d867cd564f958e7406b7ef893c9

commit 36d91d332d9c8d867cd564f958e7406b7ef893c9
Author: tapted <tapted@chromium.org>
Date: Thu Jun 22 00:34:19 2017

Mac: Don't replace the alpha on the detached toolbar separator color.

The Cocoa browser used to clobber the theme's alpha with 50%. But since
r480275, theme_service.cc makes appropriate adjustments to the color
instead.

BUG= 734476 

Review-Url: https://codereview.chromium.org/2946183002
Cr-Commit-Position: refs/heads/master@{#481375}

[modify] https://crrev.com/36d91d332d9c8d867cd564f958e7406b7ef893c9/chrome/browser/ui/cocoa/bookmarks/bookmark_bar_toolbar_view.mm

Above issue seems to be fixed to Latest Dev Version:61.0.3141.0 (Official Build) 180095eb1bca7df1cdcb02547340499c2ee3af6e-refs/heads/master@{#482153} and Latest Canary Version:61.0.3142.0 (Official Build) using Mac Pro (10.12.3 & 10.11.6)