Standardize usage of virtual/override/final in gin/

gin/per_context_data.h

Index: gin/per_context_data.h
diff --git a/gin/per_context_data.h b/gin/per_context_data.h
index 0d1165345df98d083ad1bacfe5244e34a7a4cd1c..de8f1793ef6ccc9e962c44cb2e15ebd615cd1480 100644
--- a/gin/per_context_data.h
+++ b/gin/per_context_data.h
@@ -24,7 +24,7 @@ class GIN_EXPORT PerContextData : public base::SupportsUserData {
  public:
   PerContextData(ContextHolder* context_holder,
                  v8::Handle<v8::Context> context);
-  virtual ~PerContextData();
+  ~PerContextData() override;
 
   // Can return NULL after the ContextHolder has detached from context.
   static PerContextData* From(v8::Handle<v8::Context> context);