webui: increase open externally icon contrast

This will put this icon back above contrast minimum (4.5):

9AA0A6 on FFFFFF = 2.64
5F6368 on FFFFFF = 6.04

Bug: 957394
Change-Id: I49a3f7dd30d06b140f1074651338122f84149c31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1612842
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660046}