Don't print warning if we get a SeekMap when reading DASH chunks.

Both FragmentedMp4Extractor and WebmExtractor now invoke seekMap() with
SeekMap.UNSEEKABLE if there isn't an index in the stream, so DASH playbacks
will end up printing this warning every 5 seconds or so. This is obviously
not desirable, so this CL just removes the warning! The alternative would
have been to print a warning if the value is anything other than UNSEEKABLE,
but it doesn't really seem worth it.

library/src/main/java/com/google/android/exoplayer/chunk/ContainerMediaChunk.java

@@ -27,17 +27,13 @@
 import com.google.android.exoplayer.util.ParsableByteArray;
 import com.google.android.exoplayer.util.Util;
 
-import android.util.Log;
-
 import java.io.IOException;
 
 /**
  * A {@link BaseMediaChunk} that uses an {@link Extractor} to parse sample data.
  */
 public class ContainerMediaChunk extends BaseMediaChunk implements SingleTrackOutput {
 
-  private static final String TAG = "ContainerMediaChunk";
-
   private final ChunkExtractorWrapper extractorWrapper;
   private final long sampleOffsetUs;
 
@@ -96,7 +92,7 @@ public DrmInitData getDrmInitData() {
 
   @Override
   public void seekMap(SeekMap seekMap) {
-    Log.w(TAG, "Ignoring unexpected seekMap");
+    // Do nothing.
   }
 
   @Override