WebUI Dark Mode: update incognito icon for light/dark mode

Screenshots: https://imgur.com/a/juHC3Fz

R=dpapad@chromium.org
BUG=936977

Change-Id: Ibef2af96df3599b79d3c94e979f8039a2cea42f0
Reviewed-on: https://chromium-review.googlesource.com/c/1496295
Commit-Queue: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636649}
6 files changed