Bug 1547784 - Return an error if an EME associated MediaElement becomes inactive as deocder setup finishes. r=alwu

This fixes an edge case where it was possible for an HTMLMediaElement in the
middle of setup to have ownership transferred to a inactive document and deref a
null pointer. This happened because we have special handling for EME related
media where we perform more aggressive shutdown on for media in inactive
documents.

As far as I can tell, there's nothing specced that forbids performing EME
related functionality on elements in inactive documents. However, our code
already prevents doing so in other cases. E.g. if you create an inactive
document, place an HTMLMediaElement in it and try to setup EME related data on
it, then that will fail. So this fix just covers another such case.

While it would be nice to support doing these operations on inactive document's
media, it seems like very much an edge case, and something that would require a
large amount of reworking in how we handle inactive documents. We can cross that
bridge later should we ever need do so.

Differential Revision: https://phabricator.services.mozilla.com/D40482

dom/html/HTMLMediaElement.cpp

@@ -4670,6 +4670,14 @@ nsresult HTMLMediaElement::FinishDecoderSetup(MediaDecoder* aDecoder) {
   // This will also do an AddRemoveSelfReference.
   NotifyOwnerDocumentActivityChanged();
 
+  if (!mDecoder) {
+    // NotifyOwnerDocumentActivityChanged may shutdown the decoder if the
+    // owning document is inactive and we're in the EME case. We could try and
+    // handle this, but at the time of writing it's a pretty niche case, so just
+    // bail.
+    return NS_ERROR_FAILURE;
+  }
+
   if (mPausedForInactiveDocumentOrChannel) {
     mDecoder->Suspend();
   }