Fix BrowsingDataCounter race condition

components/browsing_data/core/counters/browsing_data_counter.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_BROWSING_DATA_CORE_COUNTERS_BROWSING_DATA_COUNTER_H_
 #define COMPONENTS_BROWSING_DATA_CORE_COUNTERS_BROWSING_DATA_COUNTER_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 110 matching lines @@
   base::Time GetPeriodStart();
 
   // Returns if this counter belongs to a preference on the default, basic or
   // advanced CBD tab.
   ClearBrowsingDataTab GetTab() const;
 
  private:
   // Called after the class is initialized by calling |Init|.
   virtual void OnInitialized();
 
-  // Count the data.
+  // Count the data. Call ReportResult() when finished. Tasks that are still
+  // running should be cancelled to avoid reporting old results.
   virtual void Count() = 0;
 
   // State transition methods.
   void TransitionToShowCalculating();
   void TransitionToReadyToReportResult();
 
   // Indicates if this counter belongs to a preference on the basic CBD tab.
   ClearBrowsingDataTab clear_browsing_data_tab_;
 
   // The callback that will be called when the UI should be updated with a new
@@ skipping 21 matching lines @@
   std::unique_ptr<Result> staged_result_;
 
   // A timer to time the RESTARTED->SHOW_CALCULATION and
   // SHOW_CALCULATION->READY_TO_REPORT_RESULT state transitions.
   base::OneShotTimer timer_;
 };
 
 }  // namespace browsing_data
 
 #endif  // COMPONENTS_BROWSING_DATA_CORE_COUNTERS_BROWSING_DATA_COUNTER_H_