Add more missing closing namespace comments.

third_party/WebKit/Source/core/timing/PerformanceObserverCallback.h

 // Copyright 2015 The Chromium 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 PerformanceObserverCallback_h
 #define PerformanceObserverCallback_h
 
 #include "platform/heap/Handle.h"
 #include "wtf/RefPtr.h"
 
 namespace blink {
 
 class ExecutionContext;
 class PerformanceObserverEntryList;
 class PerformanceObserver;
 
 class PerformanceObserverCallback : public GarbageCollectedFinalized<PerformanceObserverCallback> {
 public:
     virtual ~PerformanceObserverCallback() { }
 
     virtual void handleEvent(PerformanceObserverEntryList*, PerformanceObserver*) = 0;
     virtual ExecutionContext* executionContext() const = 0;
 
     DEFINE_INLINE_VIRTUAL_TRACE() { }
 };
 
-}
+} // namespace blink
 
 #endif // PerformanceObserverCallback_h