Update virtual/final usage in tools/gn/.

Also some #include fixes (from OVERRIDE -> override conversion).

BUG=417463
[email protected]

Review URL: https://codereview.chromium.org/643063004

Cr-Commit-Position: refs/heads/master@{#300635}
diff --git a/tools/gn/target.h b/tools/gn/target.h
index b949978a012..8e5a3e2 100644
--- a/tools/gn/target.h
+++ b/tools/gn/target.h
@@ -10,7 +10,6 @@
 #include <vector>
 
 #include "base/basictypes.h"
-#include "base/compiler_specific.h"
 #include "base/logging.h"
 #include "base/strings/string_piece.h"
 #include "base/synchronization/lock.h"
@@ -52,15 +51,15 @@
   typedef std::vector<std::string> StringVector;
 
   Target(const Settings* settings, const Label& label);
-  virtual ~Target();
+  ~Target() override;
 
   // Returns a string naming the output type.
   static const char* GetStringForOutputType(OutputType type);
 
   // Item overrides.
-  virtual Target* AsTarget() override;
-  virtual const Target* AsTarget() const override;
-  virtual bool OnResolved(Err* err) override;
+  Target* AsTarget() override;
+  const Target* AsTarget() const override;
+  bool OnResolved(Err* err) override;
 
   OutputType output_type() const { return output_type_; }
   void set_output_type(OutputType t) { output_type_ = t; }