Bug 1185886: P1. Ensure DrainComplete() is called when draining. r=cpearce

If no samples were ever passed to the H264 wrapper, no decoder would have been created.

dom/media/platforms/wrappers/H264Converter.cpp

@@ -95,6 +95,7 @@ H264Converter::Drain()
   if (mDecoder) {
     return mDecoder->Drain();
   }
+  mCallback->DrainComplete();
   return mLastError;
 }