Remove "Diamond Problem" with MediaDecoder inheritance

MediaDecoder inherited from nsIObserver which inherits from nsISupport while also inheriting from AbstractMediaDecoder which itself inherits from nsISupport. This causes a "diamond problem", and prevents using some utility classes like NS_ProxyRelease or nsMainThreadPtrHandle. MediaDecoder will assert if destructed on anything but the main thread.

dom/media/AbstractMediaDecoder.h

@@ -41,7 +41,7 @@ enum class MediaDecoderEventVisibility : int8_t {
  * The AbstractMediaDecoder class describes the public interface for a media decoder
  * and is used by the MediaReader classes.
  */
-class AbstractMediaDecoder : public nsISupports
+class AbstractMediaDecoder : public nsIObserver
 {
 public:
   // Returns the monitor for other threads to synchronise access to
@@ -150,6 +150,10 @@ public:
     AbstractMediaDecoder* mDecoder;
   };
 
+  // Classes directly inheriting from AbstractMediaDecoder do not support
+  // Observe and it should never be called directly.
+  NS_IMETHOD Observe(nsISupports *aSubject, const char * aTopic, const char16_t * aData) override
+  { MOZ_CRASH("Forbidden method"); return NS_OK; }
 };
 
 class MetadataContainer

dom/media/MediaDecoder.h

@@ -264,8 +264,7 @@ struct SeekTarget {
   MediaDecoderEventVisibility mEventVisibility;
 };
 
-class MediaDecoder : public nsIObserver,
-                     public AbstractMediaDecoder
+class MediaDecoder : public AbstractMediaDecoder
 {
 public:
   struct SeekResolveValue {