Update {virtual,override} to follow C++11 style in base.

base/trace_event/trace_event.h

Index: base/trace_event/trace_event.h
diff --git a/base/trace_event/trace_event.h b/base/trace_event/trace_event.h
index 7c4d3abe27cf64a6194456db1b72b9f6b686d305..79ec4a8b85037a70532f9e707dea5a7b9fcb2bee 100644
--- a/base/trace_event/trace_event.h
+++ b/base/trace_event/trace_event.h
@@ -144,11 +144,11 @@
 //   class MyData : public base::trace_event::ConvertableToTraceFormat {
 //    public:
 //     MyData() {}
-//     virtual void AppendAsTraceFormat(std::string* out) const override {
+//     void AppendAsTraceFormat(std::string* out) const override {
 //       out->append("{\"foo\":1}");
 //     }
 //    private:
-//     virtual ~MyData() {}
+//     ~MyData() override {}
 //     DISALLOW_COPY_AND_ASSIGN(MyData);
 //   };
 //