arc: Clear the ARC task list when ARC get disabled.

chrome/browser/task_management/providers/arc/arc_process_task_provider.cc

 // 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.
 
 #include "chrome/browser/task_management/providers/arc/arc_process_task_provider.h"
 
 #include <stddef.h>
 
 #include <queue>
 #include <set>
@@ skipping 74 matching lines @@
     // Stale arc process.
     NotifyObserverTaskRemoved(nspid_to_task_[entry].get());
     nspid_to_task_.erase(entry);
   }
   ScheduleNextRequest();
 }
 
 void ArcProcessTaskProvider::RequestProcessList() {
   arc::ArcProcessService* arc_process_service =
       arc::ArcProcessService::Get();
+  auto callback = base::Bind(&ArcProcessTaskProvider::OnUpdateProcessList,
+                             weak_ptr_factory_.GetWeakPtr());
   if (!arc_process_service ||
-      !arc_process_service->RequestProcessList(
-          base::Bind(&ArcProcessTaskProvider::OnUpdateProcessList,
-                     weak_ptr_factory_.GetWeakPtr()))) {
+      !arc_process_service->RequestProcessList(callback)) {
     VLOG(2) << "ARC process instance is not ready.";
-    ScheduleNextRequest();
+    // Update the process list with an empty list of ARC processes.
+    callback.Run(std::vector<ArcProcess>());

[cylee, 2016/06/08 09:58:01]

Could you add a comment to make it clear the case happens either when ARC is not
ready or ARC has been shutdown/disabled?) 

[Shuhei Takahashi, 2016/06/08 10:50:21]

Done.

   }
 }
 
 void ArcProcessTaskProvider::StartUpdating() {
   is_updating_ = true;
   RequestProcessList();
 }
 
 void ArcProcessTaskProvider::StopUpdating() {
   is_updating_ = false;
   nspid_to_task_.clear();
 }
 
 void ArcProcessTaskProvider::ScheduleNextRequest() {
   if (!is_updating_)
     return;
   // TODO(nya): Remove this timer once ARC starts to send us UpdateProcessList
   // message when the process list changed. As of today, ARC does not send
   // the process list unless we request it by RequestProcessList message.
   base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
       FROM_HERE,
       base::Bind(&ArcProcessTaskProvider::RequestProcessList,
                  weak_ptr_factory_.GetWeakPtr()),
       base::TimeDelta::FromSeconds(kUpdateProcessListDelaySeconds));
 }
 
 }  // namespace task_management