Inherit EventTarget interface instead of duplicating its code

Source/modules/encryptedmedia/MediaKeySession.h

Index: Source/modules/encryptedmedia/MediaKeySession.h
diff --git a/Source/modules/encryptedmedia/MediaKeySession.h b/Source/modules/encryptedmedia/MediaKeySession.h
index c055c022cd37ce69b572fa81d436c5534fb7c80a..74a76bea9c925ded7930d22b925b28f363cc690b 100644
--- a/Source/modules/encryptedmedia/MediaKeySession.h
+++ b/Source/modules/encryptedmedia/MediaKeySession.h
@@ -51,7 +51,7 @@ class ContentDecryptionModuleSession;
 // it may outlive any references to it as long as the MediaKeys object is alive.
 // The ContentDecryptionModuleSession has the same lifetime as this object.
 class MediaKeySession
-    : public RefCounted<MediaKeySession>, public ScriptWrappable, public EventTarget, public ContextLifecycleObserver
+    : public EventTarget, public RefCounted<MediaKeySession>, public ScriptWrappable, public ContextLifecycleObserver
     , private ContentDecryptionModuleSessionClient {
 public:
     static PassRefPtr<MediaKeySession> create(ScriptExecutionContext*, ContentDecryptionModule*, MediaKeys*);