[presubmit] Enable readability/namespace linter checking.

src/extensions/gc-extension.h

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_EXTENSIONS_GC_EXTENSION_H_
 #define V8_EXTENSIONS_GC_EXTENSION_H_
 
 #include "include/v8.h"
 #include "src/utils.h"
 
@@ skipping 12 matching lines @@
  private:
   static const char* BuildSource(char* buf, size_t size, const char* fun_name) {
     SNPrintF(Vector<char>(buf, static_cast<int>(size)),
              "native function %s();", fun_name);
     return buf;
   }
 
   char buffer_[50];
 };
 
-} }  // namespace v8::internal
+}  // namespace internal
+}  // namespace v8
 
 #endif  // V8_EXTENSIONS_GC_EXTENSION_H_