Fixed issue in which MediaChunk.seekToStart might not actually seek to the start of the segment.

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

@@ -73,9 +73,7 @@ public abstract class MediaChunk extends Chunk {
   /**
    * Seeks to the beginning of the chunk.
    */
-  public final void seekToStart() {
-    seekTo(startTimeUs, false);
-  }
+  public abstract void seekToStart();
 
   /**
    * Seeks to the specified position within the chunk.

library/src/main/java/com/google/android/exoplayer/chunk/Mp4MediaChunk.java

@@ -54,6 +54,12 @@ public final class Mp4MediaChunk extends MediaChunk {
     this.sampleOffsetUs = sampleOffsetUs;
   }
 
+  @Override
+  public void seekToStart() {
+    extractor.seekTo(0, false);
+    resetReadPosition();
+  }
+
   @Override
   public boolean seekTo(long positionUs, boolean allowNoop) {
     long seekTimeUs = positionUs + sampleOffsetUs;

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

@@ -109,6 +109,11 @@ public class SingleSampleMediaChunk extends MediaChunk {
     return true;
   }
 
+  @Override
+  public void seekToStart() {
+    resetReadPosition();
+  }
+
   @Override
   public boolean seekTo(long positionUs, boolean allowNoop) {
     resetReadPosition();

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

@@ -50,6 +50,11 @@ public final class WebmMediaChunk extends MediaChunk {
     this.extractor = extractor;
   }
 
+  @Override
+  public void seekToStart() {
+    seekTo(0, false);
+  }
+
   @Override
   public boolean seekTo(long positionUs, boolean allowNoop) {
     boolean isDiscontinuous = extractor.seekTo(positionUs, allowNoop);