Match background color in ambient animations.

When the device is in portrait mode, we are currently using the
landscape version of the animation. This means the landscape animation's
width gets scaled down to match the width of the portrait screen and
it's center-aligned, leaving empty space on the top and bottom of the
screen. To make this less obvious to the user, fill the empty space with
a background color that exactly matches that of the animation.

This is a solution for launch. It may change in the future if we have
time to make portrait-specific versions of the animation themes.

Bug: b:217271296

Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: Iadb5e4d6185fcabbdf6b92a994779706c4d0b15a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3625594


Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Eric Sum <esum@google.com>
Reviewed-by: Jason Thai <jasontt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#999817}