Today, if we try to load `about:home`, we get a load error because it is
not a valid about page.
For an embedder to have `about:home` in the back-forward history, we
need to allow the request to load successfully. That means that we need
to let it, at minimum, do nothing - for this, we just load
`about:blank`.
Should this be behind a pref or GeckoRuntimeSetting?
Our only prior art is `about:config` which controls Gecko functionality.
This is a feature that has valid use cases for controlling behind a
pref, whereas `about:home` does nothing similar to `about:blank` which
does not have a pref.
Differential Revision: https://phabricator.services.mozilla.com/D246543
a41c51ac84