[CRD iOS] Fix empty closure crash in AudioStreamConsumer::ProcessAudioPacket

WebRTCAudioSinkAdapter passes an empty done closure to AudioStreamConsumer.
Different from a DoNothing closure, calling an empty closure will end up
crashing the app. This CL fixes AudioStreamConsumer so that it only runs
the done closure if it is not null.

Bug: 748710
Change-Id: Ied6301b54e694f8d62618e7ed4e16fa2811a4202
Reviewed-on: https://chromium-review.googlesource.com/765010Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515771}

remoting/client/audio/audio_stream_consumer.cc

@@ -10,7 +10,9 @@ void AudioStreamConsumer::ProcessAudioPacket(
     std::unique_ptr<AudioPacket> packet,
     const base::Closure& done) {
   AddAudioPacket(std::move(packet));
-  done.Run();
+  if (done) {
+    done.Run();
+  }
 }
 
 }  // namespace remoting