[Chromecast] Fix use-after-move error

Bug: internal b/145558823
Change-Id: I0bfa60287df3cbce321efac39e60da417cd9c401
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1960511
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Auto-Submit: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723477}

chromecast/media/audio/mixer_service/control_connection.cc

@@ -110,7 +110,8 @@ void ControlConnection::SetStreamCountCallback(StreamCountCallback callback) {
   stream_count_callback_ = std::move(callback);
   if (socket_) {
     Generic message;
-    message.mutable_request_stream_count()->set_subscribe(!callback.is_null());
+    message.mutable_request_stream_count()->set_subscribe(
+        !stream_count_callback_.is_null());
     socket_->SendProto(message);
   }
 }