The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9f95184bf7091b20b7ea491d9fbb29ce0f520171

commit 9f95184bf7091b20b7ea491d9fbb29ce0f520171
Author: Mark Cogan <marq@google.com>
Date: Thu Jun 07 23:48:56 2018

[iOS] Tweak tab grid segmented control backgrounds.

This CL makes the background separators for the segmented control
transparent (by making the background view itself transparent). It also
adds an alpha parameter for the background color in anticipation of
making the background itself somewhat transparent.

Bug:  849647 ,  849646 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ia17c32297a73de9de2a4903a84829f6c5f4dae75
Reviewed-on: https://chromium-review.googlesource.com/1091536
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565469}
[modify] https://crrev.com/9f95184bf7091b20b7ea491d9fbb29ce0f520171/ios/chrome/browser/ui/tab_grid/tab_grid_page_control.mm

Martijn, please review the current state of the segmented control separators and background and mark this as Fixed if they look good, or supply details of desired changes and reassign to me.

LGTM