[realbox][cr23] Fix fallback color vars when theming flag is off.

The hard-coded colors for these items that were changed the using
fallbacks in crrev.com/c/5075915 were broken because all of these vars
were set in chrome_color_id.h meaning that they got set to a default
color and were never actually set when the theming flag was off. This
CL adds them to the new tab page color mixer so they get set to their
hard-coded colors even when the theming flag is off.

Before (theming flag off):
https://screenshot.googleplex.com/B4uhwuAiUgbraX4
After (theming flag off):
https://screenshot.googleplex.com/4bhwK5R5e9YoiBd

Bug: 1513449
Change-Id: Ie8172059baf66354b3b78d7d1ecbe24886bf99e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5146208
Commit-Queue: Nihar Majmudar <niharm@google.com>
Reviewed-by: Marlon Facey <mfacey@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1240367}