Adds implementation of BackgroundScheduler.unschedule()

chrome/android/java/src/org/chromium/chrome/browser/offlinepages/BackgroundScheduler.java

 // Copyright 2016 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.chrome.browser.offlinepages;
 
 import android.content.Context;
 import android.os.Bundle;
 
 import com.google.android.gms.gcm.GcmNetworkManager;
@@ skipping 31 matching lines @@
                 .setExtras(taskExtras)
                 .build();
 
         gcmNetworkManager.schedule(task);
     }
 
     /**
      * Cancel any outstanding GCM Network Manager requests.
      */
     public static void unschedule(Context context) {
-        // TODO(petewil): Take our task off the task queue.
+        // Get the GCM Network Scheduler.
+        GcmNetworkManager gcmNetworkManager = GcmNetworkManager.getInstance(context);
+        gcmNetworkManager.cancelTask(OfflinePageUtils.TASK_TAG, ChromeBackgroundService.class);
     }
 }