1725257 - Introduce about:unloads (chrome://discards equivalent)
We're reviving automatic tab unloader feature (bug 1587762). It might be useful to have a page like chrome://discards in Chrome, especially to see which tab will be unloaded next because the tab selection logic is complicated.
Firefox has about:performance which is the task manager but shows you the energy impact rather than the CPU usage found in about:processes, which is much more useful. Neither page let you kill pages or extensions like Chrome which I opened a feature request for. And neither page has the usual design paradigm of a context menu on column headers to hide and show columns.
getAllProcesses() adds a per-tab map to hold the processes backed by the process to a tab so that we don't need to iterateprocesses twice.
Consider a process that hosts multiple frames in a single tab as a unique process because such processes are terminated when that single tab is unloaded.
https://hg.mozilla.org/mozilla-central/rev/98586fa85ecf https://hg.mozilla.org/mozilla-central/rev/061cbd7639c4 https://hg.mozilla.org/mozilla-central/rev/4e62d5d8f501
