Standardize usage of virtual/override/final in gin/

gin/array_buffer.h

Index: gin/array_buffer.h
diff --git a/gin/array_buffer.h b/gin/array_buffer.h
index 048a35fc61935478e61a3cf3b02010ac59579524..fe3fae28153e37e4c80c2fc4b2457a45ad567623 100644
--- a/gin/array_buffer.h
+++ b/gin/array_buffer.h
@@ -16,9 +16,9 @@ namespace gin {
 
 class ArrayBufferAllocator : public v8::ArrayBuffer::Allocator {
  public:
-  virtual void* Allocate(size_t length) override;
-  virtual void* AllocateUninitialized(size_t length) override;
-  virtual void Free(void* data, size_t length) override;
+  void* Allocate(size_t length) override;
+  void* AllocateUninitialized(size_t length) override;
+  void Free(void* data, size_t length) override;
 
   GIN_EXPORT static ArrayBufferAllocator* SharedInstance();
 };