We want to disable support Large-Allocation header for Fission. kmag says we want to remove it after Fission.

Deferring to Fission Beta (M7) because disabling the Large-Allocation header does not need to block enabling in Nightly (M6).

While working on process switch code for bug 1640019, I frequently ran into my old Large-Allocation code as an annoying edge case in the logic. In addition, when I originally wrote Large-Allocation support, DocumentChannel and the other process-switching tools we added for Fission were not available, so it ended up being quite complicated.

I'm morphing this bug to both disable Large-Allocation when Fission is enabled, as well as migrate the implementation from the previous reloadInFreshProcess implementation to instead using DocumentLoadListener.

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.

Backed out for bc failures on browser_autoOpen.js

Backout link: https://hg.mozilla.org/integration/autoland/rev/a86c59085c380d03391f3ecf40807d616107cd02
Log link: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=306154280&repo=autoland&lineNumber=2066

Also:

1. bc failures on browser_UsageTelemetry_content.js -> https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=306151388&repo=autoland&lineNumber=4478
2. dt failure on browser_preloaded_newtab.js -> https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=306151350&repo=autoland&lineNumber=10882

https://hg.mozilla.org/mozilla-central/rev/006268679079
https://hg.mozilla.org/mozilla-central/rev/d85bd37494ec