Reinstate buffer discard for SmoothStreaming

Now we have a ChunkSampleStream.discardBuffer, we need to call
it from SsMediaPeriod as well as for the DASH case.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175675427

library/smoothstreaming/src/main/java/com/google/android/exoplayer2/source/smoothstreaming/SsMediaPeriod.java

@@ -139,7 +139,9 @@ import java.util.ArrayList;
 
   @Override
   public void discardBuffer(long positionUs) {
-    // Do nothing.
+    for (ChunkSampleStream<SsChunkSource> sampleStream : sampleStreams) {
+      sampleStream.discardBuffer(positionUs);
+    }
   }
 
   @Override