a11y: Fix toolbar dark mode font color

In dark mode, the placeholder text got its color implicitly from the
parent toolbar element's dark mode settings. So instead, this change
explicitly defines the placeholder text's preferred dark mode color.

Before: http://screen/3ogcXSwcRfNLQAG
After: http://screen/5jHByt6gXfuTrPd

Fixed: 1146343
Change-Id: Ibd8342be6e440d9e828be6ed56dc86847fe332fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2529691Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825942}

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html

@@ -57,6 +57,7 @@
       @media (prefers-color-scheme: dark) {
         #prompt {
           --cr-toolbar-search-field-prompt-opacity: 1;
+          color: var(--cr-secondary-text-color, white);
         }
       }