Allow posting the CronetEngine UI Thread initialization on a custom Ui executor instead of the defa…

components/cronet/android/java/src/org/chromium/net/impl/CronetLibraryLoader.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.impl;
 
 import android.content.Context;
 import android.os.Handler;
 import android.os.Looper;
 
@@ skipping 51 matching lines @@
                     public void run() {
                         ensureInitializedOnMainThread(applicationContext);
                     }
                 };
                 // Run task immediately or post it to the UI thread.
                 if (Looper.getMainLooper() == Looper.myLooper()) {
                     task.run();
                 } else {
                     // The initOnMainThread will complete on the main thread prior
                     // to other tasks posted to the main thread.
-                    new Handler(Looper.getMainLooper()).post(task);
+                    if (builder.uiExecutor() != null) {
+                        builder.uiExecutor().execute(task);
+                    } else {
+                        new Handler(Looper.getMainLooper()).post(task);
+                    }
                 }
             }
         }
     }
 
     /**
      * Ensure that the main thread initialization has completed. Can only be called from
      * the main thread. Ensures that the NetworkChangeNotifier is initialzied and the
      * main thread native MessageLoop is initialized.
      */
@@ skipping 15 matching lines @@
         // the undesired initial network change observer notification, which
         // will cause active requests to fail with ERR_NETWORK_CHANGED.
         nativeCronetInitOnMainThread();
         sMainThreadInitDone = true;
     }
 
     // Native methods are implemented in cronet_library_loader.cc.
     private static native void nativeCronetInitOnMainThread();
     private static native String nativeGetCronetVersion();
 }