Fix <selectmenu> button text color

This CL:
- Makes the selected-value slot inherit its text color from the
  button slot.
- Changes the button slot color to match the <select> text colour.

BUG=1468548

Change-Id: I31cbb762b82181a27596d0902d2eeb8fdf9cd98a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4727009
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1176750}

html/semantics/forms/the-selectmenu-element/support/fake-selectmenu.js

@@ -24,7 +24,7 @@ function createFakeSelectmenu(selectedValueText) {
     }
 
     .fake-selectmenu-selected-value {
-      color: HighlightText;
+      color: FieldText;
       flex-grow:1;
     }