[fuchsia] Fix a receiver crash on a second Cast Streaming offer message

A Cast Streaming Session can have subsequent offer messages following
the initial offer. This CL ignores subsequent offer messages to
prevent a browser crash.

Bug: 1116185
Change-Id: Id41b7012d5669996ba81ed9e416ceaf7d1f60b8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357675
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Auto-Submit: Fabrice de Gans-Riberi <fdegans@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798290}

fuchsia/cast_streaming/cast_streaming_session.cc

@@ -79,6 +79,11 @@ class CastStreamingSession::Internal
     DVLOG(1) << __func__;
     DCHECK_EQ(session, &receiver_session_);
 
+    if (initialized_called_) {
+      // TODO(crbug.com/1116185): Handle multiple offer messages properly.
+      return;
+    }
+
     base::Optional<AudioStreamInfo> audio_stream_info;
     if (receivers.audio) {
       // Creare the audio data pipe.