Settings WebUI: Make textarea underline static to avoid cropping scrollbar

Bug: 956954
Change-Id: I6478ba316c02b46e93c30135e6f7077f680d57dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609595Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659712}

chrome/browser/resources/settings/controls/settings_textarea.html

@@ -14,6 +14,10 @@
       #input-container {
         background-color: var(--cr-input-background-color);
       }
+
+      #underline {
+        position: static;
+      }
     </style>
     <div id="label" hidden="[[!label]]">[[label]]</div>
     <div id="input-container">