[Cronet] Add ExperimentalCronetEngine.Builder.setThreadPriority() API

components/cronet/android/java/src/org/chromium/net/impl/JavaCronetEngineBuilderImpl.java

 // Copyright 2017 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.impl;
 
 import android.content.Context;
 
 import org.chromium.net.ExperimentalCronetEngine;
 import org.chromium.net.ICronetEngineBuilder;
 
 /**
  * Implementation of {@link ICronetEngineBuilder} that builds Java-based Cronet engine.
  */
 public class JavaCronetEngineBuilderImpl extends CronetEngineBuilderImpl {
     /**
      * Builder for Platform Cronet Engine.
      *
      * @param context Android {@link Context} for engine to use.
      */
     public JavaCronetEngineBuilderImpl(Context context) {
         super(context);
     }
 
     @Override
     public ExperimentalCronetEngine build() {
         if (getUserAgent() == null) {
             setUserAgent(getDefaultUserAgent());
         }
-        return new JavaCronetEngine(getUserAgent());
+        return new JavaCronetEngine(this);
     }
 }