Standardize usage of virtual/override/final in tools/gn/

tools/gn/ninja_group_target_writer.h

Index: tools/gn/ninja_group_target_writer.h
diff --git a/tools/gn/ninja_group_target_writer.h b/tools/gn/ninja_group_target_writer.h
index 31625f8f5f41b8766eb3b05e03b3ee45c11fb609..8f0fae68dae914d4ee98f29b64ca502b36f67b0a 100644
--- a/tools/gn/ninja_group_target_writer.h
+++ b/tools/gn/ninja_group_target_writer.h
@@ -12,9 +12,9 @@
 class NinjaGroupTargetWriter : public NinjaTargetWriter {
  public:
   NinjaGroupTargetWriter(const Target* target, std::ostream& out);
-  virtual ~NinjaGroupTargetWriter();
+  ~NinjaGroupTargetWriter() override;
 
-  virtual void Run() OVERRIDE;
+  void Run() OVERRIDE;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(NinjaGroupTargetWriter);