make ContentChunk retainable so multipart can use the chunks

Signed-off-by: Lachlan Roberts <[email protected]>

runtime/runtime_impl_jetty12/src/main/java/com/google/apphosting/runtime/jetty/delegate/impl/ContentChunk.java

@@ -18,17 +18,15 @@
 
 import java.nio.ByteBuffer;
 import java.util.Objects;
+
 import org.eclipse.jetty.io.Content;
+import org.eclipse.jetty.io.Retainable;
 import org.eclipse.jetty.util.BufferUtil;
 
-public class ContentChunk implements Content.Chunk {
+public class ContentChunk extends Retainable.ReferenceCounter implements Content.Chunk {
   private final ByteBuffer byteBuffer;
   private final boolean last;
 
-  public ContentChunk(ByteBuffer byteBuffer) {
-    this(byteBuffer, true);
-  }
-
   public ContentChunk(byte[] bytes) {
     this(BufferUtil.toBuffer(bytes), true);
   }
@@ -48,16 +46,6 @@ public boolean isLast() {
     return last;
   }
 
-  @Override
-  public void retain() {
-    throw new UnsupportedOperationException();
-  }
-
-  @Override
-  public boolean release() {
-    return true;
-  }
-
   @Override
   public String toString() {