[presubmit] Enable readability/namespace linter checking.

src/x64/code-stubs-x64.h

 // Copyright 2011 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_X64_CODE_STUBS_X64_H_
 #define V8_X64_CODE_STUBS_X64_H_
 
 namespace v8 {
 namespace internal {
 
@@ skipping 342 matching lines @@
   class RememberedSetActionBits: public BitField<RememberedSetAction, 12, 1> {};
   class SaveFPRegsModeBits: public BitField<SaveFPRegsMode, 13, 1> {};
 
   Label slow_;
   RegisterAllocation regs_;
 
   DISALLOW_COPY_AND_ASSIGN(RecordWriteStub);
 };
 
 
-} }  // namespace v8::internal
+}  // namespace internal
+}  // namespace v8
 
 #endif  // V8_X64_CODE_STUBS_X64_H_