WebUI Dark Mode: update [default] separator colors on all pages

Fix an accidental top border on the first row of Autofill settings
section. It was there but not particularly visible in light mode.

I originally thought the default separator color was GM2 GG 700 because
I started with downloads and theres a special separator color there
(between file icon and download details).

This change makes Settings and Extensions (details view) look much much
better: https://imgur.com/a/xvrYvc8

R=dpapad@chromium.org
BUG=883049

Change-Id: I6a50ffb6cc1f35f22f1b240db8a18c32b4692568
Reviewed-on: https://chromium-review.googlesource.com/c/1428561
Commit-Queue: Dan Beam <dbeam@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625085}
5 files changed