Reland of Change the update_client task runner behavior to continue on shutdown.

components/update_client/task.h

 // 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.
 
 #ifndef COMPONENTS_UPDATE_CLIENT_TASK_H_
 #define COMPONENTS_UPDATE_CLIENT_TASK_H_
 
 #include <string>
 #include <vector>
 
 #include "base/callback.h"
 #include "base/memory/scoped_ptr.h"
 #include "components/update_client/update_client.h"
 
 namespace update_client {
 
 class Task;
 
 // Defines an abstraction for a unit of work done by the update client.
 // Each invocation of the update client API results in a task being created and
 // run. In most cases, a task corresponds to a set of CRXs, which are updated
 // together.
 class Task {
  public:
   virtual ~Task() {}
 
   virtual void Run() = 0;
 
+  // Does a best effort attempt to make a task release its resources and stop
+  // soon. It is possible that a running task may complete even if this
+  // method is called.
+  virtual void Cancel() = 0;
+
   // Returns the ids corresponding to the CRXs associated with this update task.
   virtual std::vector<std::string> GetIds() const = 0;
 };
 
 }  // namespace update_client
 
 #endif  // COMPONENTS_UPDATE_CLIENT_TASK_H_