[presubmit] Enable readability/namespace linter checking.

src/base/platform/mutex.cc

 // Copyright 2013 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.
 
 #include "src/base/platform/mutex.h"
 
 #include <errno.h>
 
 namespace v8 {
 namespace base {
@@ skipping 170 matching lines @@
   if (!TryLockNativeHandle(&native_handle_)) {
     return false;
   }
 #ifdef DEBUG
   DCHECK_LE(0, level_);
   level_++;
 #endif
   return true;
 }
 
-} }  // namespace v8::base
+}  // namespace base
+}  // namespace v8