Remove virtual destructor from Variable, mark Variable final

BUG=

Review-Url: https://codereview.chromium.org/2253503002
Cr-Commit-Position: refs/heads/master@{#38656}

src/ast/variables.h

@@ -15,7 +15,7 @@ namespace internal {
 // variables. Variables themselves are never directly referred to from the AST,
 // they are maintained by scopes, and referred to from VariableProxies and Slots
 // after binding and variable allocation.
-class Variable: public ZoneObject {
+class Variable final : public ZoneObject {
  public:
   enum Kind { NORMAL, FUNCTION, THIS, ARGUMENTS };
 
@@ -23,8 +23,6 @@ class Variable: public ZoneObject {
            InitializationFlag initialization_flag,
            MaybeAssignedFlag maybe_assigned_flag = kNotAssigned);
 
-  virtual ~Variable() {}
-
   // Printing support
   static const char* Mode2String(VariableMode mode);