Bug 1713100 - Move AvailableMemoryTracker::Init() back to NS_InitXPCOM. r=gsvelto

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

toolkit/xre/nsAppRunner.cpp

@@ -11,7 +11,6 @@
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"
-#include "mozilla/AvailableMemoryTracker.h"
 #include "mozilla/Components.h"
 #include "mozilla/FilePreferences.h"
 #include "mozilla/ChaosMode.h"
@@ -5236,10 +5235,6 @@ nsresult XREMain::XRE_mainRun() {
   }
 #endif
 
-  // AvailableMemoryTracker needs to be initialized
-  // after prefs have been loaded.
-  mozilla::AvailableMemoryTracker::Init();
-
   {
     rv = appStartup->Run();
     if (NS_FAILED(rv)) {

xpcom/build/XPCOMInit.cpp

@@ -85,6 +85,7 @@
 #include "base/message_loop.h"
 
 #include "mozilla/ipc/BrowserProcessSubThread.h"
+#include "mozilla/AvailableMemoryTracker.h"
 #include "mozilla/ClearOnShutdown.h"
 #include "mozilla/CountingAllocatorBase.h"
 #include "mozilla/UniquePtr.h"
@@ -461,6 +462,7 @@ NS_InitXPCOM(nsIServiceManager** aResult, nsIFile* aBinDirectory,
   mozilla::SharedThreadPool::InitStatics();
 
   mozilla::scache::StartupCache::GetSingleton();
+  mozilla::AvailableMemoryTracker::Init();
 
   // Notify observers of xpcom autoregistration start
   NS_CreateServicesFromCategory(NS_XPCOM_STARTUP_CATEGORY, nullptr,