[cast_streaming] Add message receipt acks to CastMessagePortImpl.

Adds missing message ack invocations which blocked flow of FIDL
messages.

The issue didn't exist prior to CL 2458877,  because the FIDL/Blink
bridge that was in place discarded flow control signals.
As a result of that CL, CastMessagePortImpl now communicates
point-to-point over FIDL with no bridges in between, and therefore
the lack of acknowledgements lead to stalls in message delivery.

Test: Manually tested Cast Streaming.
Bug: 1140743
Change-Id: I05be612c110340094725f4aeb13cfc2e8e1adeaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490552
Auto-Submit: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819631}

fuchsia/cast_streaming/cast_message_port_impl.cc

@@ -273,6 +273,11 @@ void CastMessagePortImpl::PostMessage(
              << ", namespace=" << message_namespace
              << ", message=" << str_message;
   }
+
+  // Acknowledge the message and unblock the receipt of another.
+  fuchsia::web::MessagePort_PostMessage_Result result;
+  result.set_response(fuchsia::web::MessagePort_PostMessage_Response());
+  callback(std::move(result));
 }
 
 void CastMessagePortImpl::ReceiveMessage(