Bug 1724106 - Clarify threading usage when getting media debug info. r=alwu

Differential Revision: https://phabricator.services.mozilla.com/D124104

dom/html/HTMLMediaElement.cpp

@@ -7385,6 +7385,8 @@ void HTMLMediaElement::AsyncRejectPendingPlayPromises(nsresult aError) {
 }
 
 void HTMLMediaElement::GetEMEInfo(dom::EMEDebugInfo& aInfo) {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "MediaKeys expects to be interacted with on main thread!");
   if (!mMediaKeys) {
     return;
   }