Increase text contrast of urls on chrome://accessibility

Set url text colors to #696969 to increase text contrast to 5.5:1. This
matches the text color of secondary text for the page's settings.

Bug: 873019
Change-Id: Ie97c4a084735a901b778be0d6cd07b275f83bb95
Reviewed-on: https://chromium-review.googlesource.com/1176198Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: James Wallace-Lee <jamwalla@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588717}

chrome/browser/resources/accessibility/accessibility.css

@@ -25,7 +25,7 @@ img {
 }
 
 .url {
-  color: #A0A0A0;
+  color: #696969;
 }
 
 p {