Bug 1685399 - part3 : use actual invisible state to determine if we should suspend decoding. r=padenot

There is no need for decoder to use both "document visibility" and "element's layout visibility" to determine if we should suspend decoding.

That can simply be done by checking `HTMLMediaElement::IsActuallyInvisible()`.

Differential Revision: https://phabricator.services.mozilla.com/D101108

dom/html/HTMLMediaElement.cpp

@@ -7506,7 +7506,7 @@ void HTMLMediaElement::GetEMEInfo(dom::EMEDebugInfo& aInfo) {
 
 void HTMLMediaElement::NotifyDecoderActivityChanges() const {
   if (mDecoder) {
-    mDecoder->NotifyOwnerActivityChanged(!IsHidden(), mVisibilityState,
+    mDecoder->NotifyOwnerActivityChanged(IsActuallyInvisible(),
                                          IsInComposedDoc());
   }
 }