cros: Fix icon colors for OS settings side nav and update checkmark

https://chromium-review.googlesource.com/c/chromium/src/+/2126408
changed the "cros-default-button-color" from blue to grey. However,
the side nav menu and update completed checkmark icon depended on that
color to be blue.

Change them to use cros_default_icon_color_prominent, which is the
same blue as they used to be.

Before fix: http://screen/D1rKP59xeQ9 http://screen/wpSK6N7b348
After fix:  http://screen/Tf3Mtb6KqMi http://screen/KAkaA5TGA5r

NOTE: There are still references to --cros-default-button-color in
crostini_upgrader/app.html. These may also need to be fixed. I'm just
not sure how to spawn that UI to check.

Bug: 1070423
Change-Id: I73e2bf0f98b0df5e4d6c3f36e25de5bfe9a95e06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2148251
Commit-Queue: James Cook <jamescook@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758694}

chrome/browser/resources/settings/chromeos/os_about_page/os_about_page.html

@@ -63,7 +63,7 @@
       }
 
       iron-icon[icon='settings:check-circle'] {
-        fill: var(--cros-default-button-color);
+        fill: var(--cros-default-icon-color-prominent);
       }
 
       iron-icon[icon='cr:error'] {

chrome/browser/resources/settings/chromeos/os_settings_menu/os_settings_menu.html

@@ -18,7 +18,7 @@
   <template>
     <style include="settings-shared">
       :host {
-        --menu-link-color: var(--cros-default-button-color);
+        --menu-link-color: var(--cros-default-icon-color-prominent);
         --menu-text-color: var(--cros-default-text-color-secondary);
         --menu-icon-color: var(--cros-default-text-color-secondary);
         /* The tap target extends slightly above each visible menu item. */