WebUI Dark Mode: fix toolbar cancel icon fill color

I accidentally used GG200 for the fill color (should be GG500).

R=dpapad@chromium.org

Change-Id: Ic178eaac3f254254d5bd15ee613a9cbd8e8871b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559399
Auto-Submit: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649213}

ui/webui/resources/images/dark/icon_toolbar_cancel.svg

-<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 2 4" fill="#e8eaed"><path d="M12 2C6.47 2 2 6.47 2 12s4.47 10 10 10 10-4.47 10-10S17.53 2 12 2zm5 13.59L15.59 17 12 13.41 8.41 17 7 15.59 10.59 12 7 8.41 8.41 7 12 10.59 15.59 7 17 8.41 13.41 12 17 15.59z"/></svg>
\ No newline at end of file
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 2 4" fill="#9aa0a6"><path d="M12 2C6.47 2 2 6.47 2 12s4.47 10 10 10 10-4.47 10-10S17.53 2 12 2zm5 13.59L15.59 17 12 13.41 8.41 17 7 15.59 10.59 12 7 8.41 8.41 7 12 10.59 15.59 7 17 8.41 13.41 12 17 15.59z"/></svg>