[dnt] Show content layer when navigating back away from NTP

Hide the NTP in order to show the transition drawn by the native.

Before: https://hsv.googleplex.com/5361127935442944
After: https://hsv.googleplex.com/4520320844169216

The follow-up CL will make the animator to capture the NTP view before
showing the animation and also add the tests.

Bug: 331780926
Change-Id: If8fc184d69fa8c50b614997adbaf6c9c38b9d824
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5763628


Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Lijin Shen <lazzzis@google.com>
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1343752}