Bug 1727514 - media playback should stop in the bfcache, r=peterv

This version doesn't change SetContainer handling, since it seems to be tricky for the top level page.
So only activity change notification is fired and IsActive() is updated.
The comment about IsActive() was wrong even with the old bfcache implementation.
(I did check that it returned false when the page was in bfcache and many of the activity observers rely on that)

The changes to HTMLMediaElement are needed to ensure page can enter bfcache..

Differential Revision: https://phabricator.services.mozilla.com/D124684

docshell/base/nsDocShell.cpp

@@ -1161,6 +1161,7 @@ void nsDocShell::FirePageHideShowNonRecursive(bool aShow) {
     mFiredUnloadEvent = false;
     RefPtr<Document> doc = contentViewer->GetDocument();
     if (doc) {
+      doc->NotifyActivityChanged();
       RefPtr<nsGlobalWindowInner> inner =
           mScriptGlobal ? mScriptGlobal->GetCurrentInnerWindowInternal()
                         : nullptr;