Add a few missing overrides found by a new clang warning.

src/compiler/ast-graph-builder.h

Index: src/compiler/ast-graph-builder.h
===================================================================
--- src/compiler/ast-graph-builder.h	(revision 24959)
+++ src/compiler/ast-graph-builder.h	(working copy)
@@ -56,7 +56,7 @@
   // Support for control flow builders. The concrete type of the environment
   // depends on the graph builder, but environments themselves are not virtual.
   typedef StructuredGraphBuilder::Environment BaseEnvironment;
-  virtual BaseEnvironment* CopyEnvironment(BaseEnvironment* env);
+  virtual BaseEnvironment* CopyEnvironment(BaseEnvironment* env) OVERRIDE;
 
   // TODO(mstarzinger): The pipeline only needs to be a friend to access the
   // function context. Remove as soon as the context is a parameter.
@@ -110,7 +110,7 @@
   // Builder for stack-check guards.
   Node* BuildStackCheck();
 
-#define DECLARE_VISIT(type) virtual void Visit##type(type* node);
+#define DECLARE_VISIT(type) virtual void Visit##type(type* node) OVERRIDE;
   // Visiting functions for AST nodes make this an AstVisitor.
   AST_NODE_LIST(DECLARE_VISIT)
 #undef DECLARE_VISIT