Add some missing virtual destructors to base classes.

base/trace_event/trace_event_system_stats_monitor.h

 // Copyright 2013 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 BASE_TRACE_EVENT_TRACE_EVENT_SYSTEM_STATS_MONITOR_H_
 #define BASE_TRACE_EVENT_TRACE_EVENT_SYSTEM_STATS_MONITOR_H_
 
 #include "base/base_export.h"
 #include "base/gtest_prod_util.h"
 #include "base/memory/ref_counted.h"
@@ skipping 15 matching lines @@
     : public TraceLog::EnabledStateObserver {
  public:
   // Length of time interval between stat profiles.
   static const int kSamplingIntervalMilliseconds = 2000;
 
   // |task_runner| must be the primary thread for the client
   // process, e.g. the UI thread in a browser.
   explicit TraceEventSystemStatsMonitor(
       scoped_refptr<SingleThreadTaskRunner> task_runner);
 
-  virtual ~TraceEventSystemStatsMonitor();
+  ~TraceEventSystemStatsMonitor() override;
 
   // base::trace_event::TraceLog::EnabledStateChangedObserver overrides:
   void OnTraceLogEnabled() override;
   void OnTraceLogDisabled() override;
 
   // Retrieves system profiling at the current time.
   void DumpSystemStats();
 
  private:
   FRIEND_TEST_ALL_PREFIXES(TraceSystemStatsMonitorTest,
@@ skipping 19 matching lines @@
 // Converts system memory profiling stats in |input| to
 // trace event compatible JSON and appends to |output|. Visible for testing.
 BASE_EXPORT void AppendSystemProfileAsTraceFormat(const SystemMetrics&
                                                   system_stats,
                                                   std::string* output);
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_TRACE_EVENT_SYSTEM_STATS_MONITOR_H_