Bug 1508306 - Part 1: Migate the Large-Allocation handler to DocumentLoadListener, r=mattwoodrow,necko-reviewers,geckoview-reviewers,agi,valentin

This removes the diagnostic warnings which used to be logged when the
Large-Allocation header was present, but failed to switch into a
Large-Allocation process. Due to the low adoption of the header, this shouldn't
be too large of a problem, but we can look into re-adding the diagnostics if
needed in the future.

The new codepath no longer performs multiple network requests for
Large-Allocation resources, and now relies on the battle-tested
DocumentLoadListener codepath for process switching.

Differential Revision: https://phabricator.services.mozilla.com/D78998