Inside of nsDocShell::UpdateURLAndHistory, there are 4 sync IPC calls to
nsSHistory plus 1 static call, which contains at least one nsSHEntry::GetParent
sync IPC call. All of these calls can be moved inside of a new method
EvictContentViewersOrReplaceEntry on nsSHEntry, resulting in just 1 sync IPC
call.

When we need to evict content viewers, we group SHEntrySharedParentState,
corresponding to session history entries that need to be evicted, by their
content parent, and create a runnable for each content parent. In the runnable
we use the passed in IDs of SHEntrySharedParentStates to retrieve corresponding content
viewers and evict them.

Depends on D32729

Add a new method to session history listener to listen for content
viewers being evicted.

Depends on D32730

Peter, here I will explain the rationale behind adding method NotifyListenersContentViewerEvicted to SHEntryChildShared and SHEntrySharedParentState. But first, a backstory!

Whenever NotifyListenersContentViewerEvicted is defined on nsSHistory or SHistoryChild, it actually uses the NOTIFY_LISTENERS macro and informs mListeners about an event. I have added NotifyListenersContentViewerEvicted() in nsISHistory.idl because nsSHistory and SHistoryChild both have their own listeners. Then I added calls to NotifyListenersContentViewerEvicted() in nsSHistory::EvictContentViewerForEntry and SHistoryChild::NotifyListenersContentViewerEvicted because these two places evict content viewers.

There are several other places where a content viewer gets evicted

• SHEntryChildShared::EvictContentViewers()
• nsSHEntry::EvictContentViewer()
• SHEntryChild::EvictContentViewer()

We don't have access to session history from those places to call nsISHistory::NotifyListenersContentViewerEvicted, however, we do have access to mShared, the type of which is SHEntryChildShared or SHEntrySHaredParentState, both of which have local mSHistory. Thus, we want to define a method on SHEntryChildShared and SHEntrySHaredParentState that will invoke NotifyListenersContentViewerEvicted() on the session history. The solution is to define a method NotifyListenersContentViewerEvicted which will use local mSHistory and call nsISHistory::NotifyListenersContentViewerEvicted on it.

Pushed to ash branch

https://hg.mozilla.org/projects/ash/rev/2cd5cd24763ff320719aedb2142a79822efd6de4
https://hg.mozilla.org/projects/ash/rev/0feb286c5b85e6e2a0ac56058c596324ca28d817
https://hg.mozilla.org/projects/ash/rev/5e4d0ab9e52ce1e1d1684fdb59534a66e9f9d9b9

https://hg.mozilla.org/mozilla-central/rev/351e31cced31
https://hg.mozilla.org/mozilla-central/rev/3b772fd4f675
https://hg.mozilla.org/mozilla-central/rev/8361b46b39db