Bug 1654080 - Use PBackground for syncing SessionStorageCache and use LSWriteOptimizer to send data changes; r=dom-workers-and-storage-reviewers,janv,nika

This patch does:
- Use LSWriteOptimizer
- Remove SessionStorageService since it's unused.
- Move IPC from PContent to PBackground
(by SessionStorageManager{Child, Parent} and SessionStorageCache{Child, Parent}).
- Extract SessionStorageManagerBase and add PBackgroundSessionStorageManager.
- Expose a getter function to get a BackgroundParentManager for top context id
on the parent.

IPC
- Before this patch:
  - Copy from parent while loading a document.
    - Mark cache entry on the parent process as loaded by the child id.
  - Update change on checkpoint.
  - Unmark cache entry on the parent process as unloaded for the child id while
  the parent actor is destorying.
- After this patch:
  - Sync IPC load in the first SessionStorage operation.
  - Update change on checkpoint

`BackgroundSessionStorageManager`'s lifecycle on the parent process.
- Create by `SessionStorageManagerParent` and register to the `sManagers`.
- Hold by `SessionStorageManagerParent` and `sManagers`.
- Remove from the `sManagers` while the corresponding `BrowsingContext` is
destructed (on the parent process).

Depends on D89341

Differential Revision: https://phabricator.services.mozilla.com/D89342

docshell/base/CanonicalBrowsingContext.cpp

@@ -20,6 +20,7 @@
 #include "mozilla/dom/MediaControlService.h"
 #include "mozilla/dom/ContentPlaybackController.h"
 #include "mozilla/dom/SessionHistoryEntry.h"
+#include "mozilla/dom/SessionStorageManager.h"
 #include "mozilla/ipc/ProtocolUtils.h"
 #include "mozilla/net/DocumentLoadListener.h"
 #include "mozilla/NullPrincipal.h"
@@ -744,6 +745,10 @@ void CanonicalBrowsingContext::CanonicalDiscard() {
     mTabMediaController->Shutdown();
     mTabMediaController = nullptr;
   }
+
+  if (IsTop()) {
+    BackgroundSessionStorageManager::RemoveManager(Id());
+  }
 }
 
 void CanonicalBrowsingContext::NotifyStartDelayedAutoplayMedia() {