Fix chrome://flags search box placeholder visibility in dark mode

Default placeholder text color in input boxes was changed to a color with
transparency, which may cause placeholder text to be unreadable by default
if the input box background is a dark color. This change adds explicit
styling to the placeholder color.

Bug: 1001957
Change-Id: I7b24ab1e01a96cc5b7caa0f22ade8ab712951fe9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1799997Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Jeffrey Chen <jeffreyca16@gmail.com>
Cr-Commit-Position: refs/heads/master@{#696060}

components/flags_ui/resources/flags.css

@@ -37,6 +37,7 @@ html {
   --warning-color: var(--google-red-700);
 
   --input-background: var(--google-grey-100);
+  --input-placeholder-color: rgb(117, 117, 117);
   --keyboard-focus-ring: rgba(var(--google-blue-500-rgb), 0.4);
   --link-color: var(--google-blue-700);
   --separator-color: rgba(0, 0, 0, .06);
@@ -178,6 +179,10 @@ button {
   width: 100%;
 }
 
+#search::placeholder {
+  color: var(--input-placeholder-color);
+}
+
 @media (prefers-color-scheme: dark) {
   #search {
     background-image: url(../../../ui/webui/resources/images/dark/icon_search.svg);