Revert of [Cronet] Remove JSON serialization of CronetEngine.Builder

components/cronet/android/java/src/org/chromium/net/ChromiumUrlRequestFactory.java

 // Copyright 2014 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.content.Context;
 import android.os.Build;
 
 import org.chromium.base.annotations.UsedByReflection;
 
 import java.nio.channels.WritableByteChannel;
 import java.util.Map;
 
 /**
  * Network request factory using the native http stack implementation.
  * @deprecated Use {@link CronetEngine} instead.
  */
 @UsedByReflection("HttpUrlRequestFactory.java")
 @Deprecated
 public class ChromiumUrlRequestFactory extends HttpUrlRequestFactory {
     private ChromiumUrlRequestContext mRequestContext;
 
     @UsedByReflection("HttpUrlRequestFactory.java")
     public ChromiumUrlRequestFactory(Context context, CronetEngine.Builder config) {
         if (isEnabled()) {
             String userAgent = config.getUserAgent();
-            if (userAgent == null) {
+            if (userAgent.isEmpty()) {
                 // Cannot use config.getDefaultUserAgent() as config.mContext may be null.
                 userAgent = new CronetEngine.Builder(context).getDefaultUserAgent();
             }
             mRequestContext = new ChromiumUrlRequestContext(context,
                     userAgent, config);
         }
     }
 
     @Override
     public boolean isEnabled() {
@@ skipping 27 matching lines @@
 
     @Override
     public void stopNetLog() {
         mRequestContext.stopNetLog();
     }
 
     public ChromiumUrlRequestContext getRequestContext() {
         return mRequestContext;
     }
 }