[Android] BackgroundTaskScheduler reschedule implementation

components/background_task_scheduler/android/junit/src/org/chromium/components/background_task_scheduler/BackgroundTaskSchedulerPrefsTest.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 static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
@@ skipping 78 matching lines @@
         BackgroundTaskSchedulerPrefs.removeScheduledTask(TASK_1.getTaskId());
         assertEquals("Removing a task which is not there does not affect the task set.", 1,
                 BackgroundTaskSchedulerPrefs.getScheduledTasks().size());
 
         Set<String> scheduledTasks = BackgroundTaskSchedulerPrefs.getScheduledTasks();
         assertFalse("TASK_1 class name in scheduled tasks.",
                 scheduledTasks.contains(TASK_1.getBackgroundTaskClass().getName()));
         assertTrue("TASK_2 class name in scheduled tasks.",
                 scheduledTasks.contains(TASK_2.getBackgroundTaskClass().getName()));
     }
+
+    @Test
+    @Feature("BackgroundTaskScheduler")
+    public void testRemoveAllTasks() {
+        BackgroundTaskSchedulerPrefs.addScheduledTask(TASK_1);
+        BackgroundTaskSchedulerPrefs.addScheduledTask(TASK_2);
+        BackgroundTaskSchedulerPrefs.removeAllTasks();
+        assertTrue("We are expecting a all tasks to be gone.",
+                BackgroundTaskSchedulerPrefs.getScheduledTasks().isEmpty());
+    }
 }