Bug 1672072 - P6. Don't attempt to create a video decoder if we don't have a KnowsCompositor set. r=mattwoodrow,bryce,mjf,padenot

Differential Revision: https://phabricator.services.mozilla.com/D96362

dom/media/ipc/RemoteDecoderManagerChild.cpp

@@ -247,6 +247,13 @@ RemoteDecoderManagerChild::CreateVideoDecoder(
   }
 
   MOZ_ASSERT(aLocation != RemoteDecodeIn::Unspecified);
+
+  if (!aParams.mKnowsCompositor && aLocation == RemoteDecodeIn::GpuProcess) {
+    // We don't have an image bridge; don't attempt to decode in the GPU
+    // process.
+    return nullptr;
+  }
+
   RefPtr<RemoteVideoDecoderChild> child;
   MediaResult result(NS_ERROR_DOM_MEDIA_CANCELED);