If we're computing the stroke as (outer border - inner border), then presumably this is due to precision loss in Skia that we can't do much about but also wouldn't be visible at non-extreme scales.

Yeah I was thinking the same thing.  But it might be fixable after I land the patch that draws opaque stroke colors.  We would just need to pass 0.5*stroke_thickness to GetInteriorPath().

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6044ebe19878468f70a22d15dbef7b7987f25bf5

commit 6044ebe19878468f70a22d15dbef7b7987f25bf5
Author: Tom Anderson <thomasanderson@chromium.org>
Date: Tue Aug 14 02:26:16 2018

[Refresh] Fix frame color showing through tabs with strokes

BUG= 873003 
R=pkasting

Change-Id: I9f2550b32a94c143f443bcb7e0762bd9dde6f7ee
Reviewed-on: https://chromium-review.googlesource.com/1173652
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582801}
[modify] https://crrev.com/6044ebe19878468f70a22d15dbef7b7987f25bf5/chrome/browser/ui/views/tabs/tab.cc