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}

ui/webui/resources/images/open_in_new.svg

-<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" fill="#9AA0A6"><path d="M19 19H5V5h7V3H5a2 2 0 0 0-2 2v14a2 2 0 0 0 2 2h14c1.1 0 2-.9 2-2v-7h-2v7zM14 3v2h3.59l-9.83 9.83 1.41 1.41L19 6.41V10h2V3h-7z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" fill="#5F6368"><path d="M19 19H5V5h7V3H5a2 2 0 0 0-2 2v14a2 2 0 0 0 2 2h14c1.1 0 2-.9 2-2v-7h-2v7zM14 3v2h3.59l-9.83 9.83 1.41 1.41L19 6.41V10h2V3h-7z"/></svg>
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