Bug 1931301 - Notify of removals on a stable state, not mid-remove. r=smaug,masayuki,dshin

This simplifies some observers, and makes others a bit more subtle, but I think
over-all it is an improvement.

Differential Revision: https://phabricator.services.mozilla.com/D229008

dom/html/HTMLMediaElement.cpp

@@ -2179,10 +2179,9 @@ void HTMLMediaElement::AddSizeOfExcludingThis(nsWindowSizes& aSizes,
   }
 }
 
-void HTMLMediaElement::ContentRemoved(nsIContent* aChild,
-                                      nsIContent* aPreviousSibling) {
+void HTMLMediaElement::ContentWillBeRemoved(nsIContent* aChild) {
   if (aChild == mSourcePointer) {
-    mSourcePointer = aPreviousSibling;
+    mSourcePointer = aChild->GetPreviousSibling();
   }
 }