Make QuicSimpleServerStream supports multipart response body

components/cronet/android/test/javatests/src/org/chromium/net/TestBidirectionalStreamCallback.java

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.net;
 
 import android.os.ConditionVariable;
 
 import static junit.framework.Assert.assertEquals;
 import static junit.framework.Assert.assertFalse;
@@ skipping 200 matching lines @@
     }
 
     @Override
     public void onResponseTrailersReceived(BidirectionalStream stream, UrlResponseInfo info,
             UrlResponseInfo.HeaderBlock trailers) {
         assertEquals(mExecutorThread, Thread.currentThread());
         assertFalse(stream.isDone());
         assertNull(mError);
         mResponseStep = ResponseStep.ON_TRAILERS;
         mTrailers = trailers;
+        assertTrue(mTrailers != null);
         if (maybeThrowCancelOrPause(stream, mReadStepBlock)) {
             return;
         }
     }
 
     @Override
     public void onSucceeded(BidirectionalStream stream, UrlResponseInfo info) {
         assertEquals(mExecutorThread, Thread.currentThread());
         assertTrue(stream.isDone());
         assertTrue(mResponseStep == ResponseStep.ON_RESPONSE_STARTED
@@ skipping 94 matching lines @@
         };
         if (mFailureType == FailureType.CANCEL_ASYNC
                 || mFailureType == FailureType.CANCEL_ASYNC_WITHOUT_PAUSE) {
             getExecutor().execute(task);
         } else {
             task.run();
         }
         return mFailureType != FailureType.CANCEL_ASYNC_WITHOUT_PAUSE;
     }
 }