Bug 1522547 - Have MediaKeys observe if the owning document becomes inactive so it can shutdown. r=cpearce

MediaKeys objects are typically created and associated with an HTMLMediaElement,
however it is possible to create a MediaKeys object and not associate it with an
HTMLMediaElement.

This resulted in an issue where these MediaKeys would keep alive other
components that would assert during bowrser shutdown (see bug 1522547). We
anticipated that MediaKeys associated with an HTMLMediaElement would need to be
shutdown if their owning document became inactive, but were not handling the
case where the keys never became associated with an element.

This patch has the MediaKeys listen directly to their owning document for
activity change. The MediaKeys will shutdown if their document becomes inactive.
This avoids MediaKeys not associated with HTMLMediaElements keeping other
objects alive during browser shutdown.

Differential Revision: https://phabricator.services.mozilla.com/D21983

dom/html/HTMLMediaElement.cpp

@@ -6027,7 +6027,8 @@ void HTMLMediaElement::NotifyOwnerDocumentActivityChanged() {
 
   // If the owning document has become inactive we should shutdown the CDM.
   if (!OwnerDoc()->IsCurrentActiveDocument() && mMediaKeys) {
-    mMediaKeys->Shutdown();
+    // We don't shutdown MediaKeys here because it also listens for document
+    // activity and will take care of shutting down itself.
     DDUNLINKCHILD(mMediaKeys.get());
     mMediaKeys = nullptr;
     if (mDecoder) {