rename PerformanceServerTiming members to conform with 'Blink Rename'

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

 // Copyright 2017 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 PerformanceServerTiming_h
 #define PerformanceServerTiming_h
 
 #include "core/timing/PerformanceEntry.h"
 
 namespace blink {
@@ skipping 16 matching lines @@
 
  protected:
   void BuildJSONValue(V8ObjectBuilder&) const override;
 
  private:
   PerformanceServerTiming(const String& name,
                           const String& metric,
                           double duration,
                           const String& description);
 
-  const String m_metric;
-  const String m_description;
+  const String metric_;
+  const String description_;
 };
 
 }  // namespace blink
 
 #endif  // PerformanceServerTiming_h