Merge pull request #319 from b-ma/fix/destination-constraints

Fix `AudioDestination` constraints

Author: orottier

src/node/destination.rs

@@ -61,7 +61,12 @@ impl AudioNode for AudioDestinationNode {
         self.channel_config.set_count(v);
     }
     fn set_channel_count_mode(&self, _v: ChannelCountMode) {
-        panic!("InvalidStateError: AudioDestinationNode has channel count mode constraints");
+        // [spec] If the AudioDestinationNode is the destination node of an
+        // OfflineAudioContext, then the channel count mode cannot be changed.
+        // An InvalidStateError exception MUST be thrown for any attempt to change the value.
+        if self.registration.context().offline() {
+            panic!("InvalidStateError: AudioDestinationNode has channel count mode constraints");
+        }
     }
 }