Bug 1711610 moved `AvailableMemoryTracker::Init()` from `NS_InitXPCOM` to `XRE_mainRun`,
but it caused memory degradation because `AvailableMemoryTracker` was no longer initialized
in the child processes.
I made that part for `nsAvailableMemoryWatcher` to cache the pref value in the earlier design,
but it's not needed at all in the current design because `nsAvailableMemoryWatcher` loads
a mirror value every time.
This patch reverts `AvailableMemoryTracker::Init()` back to `NS_InitXPCOM`.
Differential Revision: https://phabricator.services.mozilla.com/D116742
a2c93279d1