bindings: Fix coding style in SequenceTest.

third_party/WebKit/Source/core/testing/SequenceTest.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 SequenceTest_h
 #define SequenceTest_h
 
 #include "bindings/core/v8/DoubleOrDoubleSequence.h"
 #include "bindings/core/v8/Nullable.h"
 #include "core/dom/Element.h"
@@ skipping 23 matching lines @@
   HeapVector<Member<Element>> getElementSequence() const;
   void setElementSequence(const HeapVector<Member<Element>>& arg);
 
   bool unionReceivedSequence(const DoubleOrDoubleSequence& arg);
 
   DECLARE_TRACE();
 
  private:
   SequenceTest();
 
-  HeapVector<Member<Element>> m_elementSequence;
+  HeapVector<Member<Element>> element_sequence_;
 };
 
 }  // namespace blink
 
 #endif  // SequenceTest_h