Add some missing virtual destructors to base classes.

base/trace_event/trace_event_impl.h

Index: base/trace_event/trace_event_impl.h
diff --git a/base/trace_event/trace_event_impl.h b/base/trace_event/trace_event_impl.h
index ccb850033e1926d59ff4396b8a9bb36acf61a109..108ae8be328b075243d5aeef9b15f49f14b05931 100644
--- a/base/trace_event/trace_event_impl.h
+++ b/base/trace_event/trace_event_impl.h
@@ -367,6 +367,8 @@ class BASE_EXPORT TraceLog : public MemoryDumpProvider {
   // on-demand.
   class BASE_EXPORT EnabledStateObserver {
    public:
+    virtual ~EnabledStateObserver() = default;
+
     // Called just after the tracing system becomes enabled, outside of the
     // |lock_|. TraceLog::IsEnabled() is true at this point.
     virtual void OnTraceLogEnabled() = 0;