Bug 1046301 - AppleATDecoder: call DrainComplete. r=cpearce

Propagate changes from bug 1039128.

content/media/fmp4/apple/AppleATDecoder.cpp

@@ -131,6 +131,7 @@ AppleATDecoder::Drain()
 {
   LOG("Draining AudioToolbox AAC decoder");
   mTaskQueue->AwaitIdle();
+  mCallback->DrainComplete();
   return Flush();
 }