Add chrome://prefs-internals to about:about

This page is useful for debugging so it should appear at about:about.
Also update the comment explaining how to do so.

Bug: None

Change-Id: I5c0ecac337381315ca8427d7046831f0f00275e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627855
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662959}