[mojo] Only allow adding valid receivers to ReceiverSet

Bug: 1174338
Change-Id: I8a78dead21f5001cc7946244a435a38b947426ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2673172
Commit-Queue: Ken Rockot <rockot@google.com>
Auto-Submit: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#850691}

mojo/public/cpp/bindings/receiver_set.h

@@ -358,6 +358,7 @@ class ReceiverSetBase {
                      PendingType receiver,
                      Context context,
                      scoped_refptr<base::SequencedTaskRunner> task_runner) {
+    DCHECK(receiver.is_valid());
     ReceiverId id = next_receiver_id_++;
     DCHECK_GE(next_receiver_id_, 0u);
     auto entry =