In Bug 1549973, we introduce a way to do the final tabState flush before closing the tab.
It can work successfully on the normal case.

However, we cannot complete the final flush when doing "await BrowserTestUtils.closeWindow(win);".
https://searchfox.org/mozilla-central/rev/8a990595ce6d5ed07ace2d4d5d86cc69aec90bde/browser/components/sessionstore/test/browser_broadcast.js#54

From the investigation, it seems that BrowserChild have sent the finsh flush by "SendSessionStoreUpdate()". However, BrowserParent cannot receive this IPC call.

Update the investigation,
If we just close the tab, the mechanism works correctly.
However, if the tab is closed by window.close(), we cannot finish the final TabState update.
The reason is we cannot get xulBrowserWindow at this time.
https://searchfox.org/mozilla-central/rev/11712bd3ce7454923e5931fa92eaf9c01ef35a0a/dom/ipc/BrowserParent.cpp#2638

Try looks fine.
https://treeherder.mozilla.org/#/jobs?repo=try&revision=8caa9a9acf516e08d953f58ee2f104e010a04ddf

Pushed by archaeopteryx@coole-files.de:
https://hg.mozilla.org/integration/autoland/rev/10986407e229
Move the sessionStore related functions from nsIXULBrowserWindow.idl r=peterv

https://hg.mozilla.org/mozilla-central/rev/10986407e229