Temporarily revert changes which cleaned up messages in the ContentSearchParent

These were removed in bug 2003294, but for bug 2017049 we may need to restore them.

This will make it easier for the work in bug 2017049.

This reverts commit aca14b8e67152e57d223b50bdf3dba99f4324d9e.

This will make it easier for the work in bug 2017049.

This reverts commit 8943ca67c923f392cbd8321c402ac4f434cd9a62.

[Tracking Requested - why for this release]:

We removed support for the non-handoff version of the New Tab search input based on the fact that it was a non-default, non-exposed, and essentially unsupported configuration - and the number of clients making searches using that mode was so small (see bug 1999334 comment 5). We've now gathered enough evidence and feedback to suggest that a significant portion of these users was using the unsupported non-handoff settings for accessibility reasons, and that the handoff mode is not accessible for them. PM has decided to let this preference limp along for a few releases further until we can complete the inline search input experience (MCAB), to reduce user-pain.

We're going to see if we can get this reversion / revival of the pref into the next planned dot release after Firefox 148 hits the release channel.

https://hg.mozilla.org/mozilla-central/rev/4de45c6a16d1
https://hg.mozilla.org/mozilla-central/rev/63c0c4214451

This will make it easier for the work in bug 2017049.

This undoes the work in commit aca14b8e67152e57d223b50bdf3dba99f4324d9e, accounting for changes made since.

Original Revision: https://phabricator.services.mozilla.com/D284294

This will make it easier for the work in bug 2017049.

This undoes the work in commit 8943ca67c923f392cbd8321c402ac4f434cd9a62, accounting for changes made since.

Original Revision: https://phabricator.services.mozilla.com/D284295

:mconley, could you add a release uplift request when you have a moment?
We can aim to include it in the Fx148 planned dot release.

It turns out these patches do not need to be uplifted. Bug 2003294, which removed the code that this bug puts back landed in 149, which is currently in Beta.

@Mathew: It is a dependency, as this wouldn't have been necessary.