[Android] Adding scheduling through GcmNetworkManager

components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskSchedulerFactory.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.components.background_task_scheduler;
 
 import android.annotation.TargetApi;
 import android.os.Build;
 
 import org.chromium.base.ThreadUtils;
 
 /**
  * A factory for {@link BackgroundTaskScheduler} that ensures there is only ever a single instance.
  */
 public final class BackgroundTaskSchedulerFactory {
     private static BackgroundTaskScheduler sInstance;
 
     @TargetApi(Build.VERSION_CODES.M)
     private static BackgroundTaskSchedulerDelegate getSchedulerDelegate() {
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
             return new BackgroundTaskSchedulerJobService();
         } else {
-            throw new RuntimeException("Not able to schedule through GCM yet.");
+            return new BackgroundTaskSchedulerGcmNetworkManager();
         }
     }
 
     /**
      * @return the current instance of the {@link BackgroundTaskScheduler}. Creates one if none
      * exist.
      */
     public static BackgroundTaskScheduler getScheduler() {
         ThreadUtils.assertOnUiThread();
         if (sInstance == null) sInstance = new BackgroundTaskScheduler(getSchedulerDelegate());
         return sInstance;
     }
 
     // Do not instantiate.
     private BackgroundTaskSchedulerFactory() {}
 }