Fix <textarea> rendering bug on about:flags in dark mode

R=rbpotter@chromium.org
BUG=970161

Change-Id: If94827f9e3b0c17450c0ed6caed2b7ecb707151f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643991
Auto-Submit: Dan Beam <dbeam@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666065}

components/flags_ui/resources/flags.css

@@ -334,6 +334,7 @@ textarea {
   background: var(--input-background);
   border-radius: 3px;
   box-sizing: border-box;
+  color: inherit;
   font-size: .8125rem;
   margin: 0;
   min-height: 3em;