Avoid calling hasAttribute() and then getAttribute() for performance

Source/core/inspector/DOMEditor.cpp

Index: Source/core/inspector/DOMEditor.cpp
diff --git a/Source/core/inspector/DOMEditor.cpp b/Source/core/inspector/DOMEditor.cpp
index 1f8956b08845319837ac1c6e0088b171e1af1b12..734077faddb14f82e73742529677da388969266d 100644
--- a/Source/core/inspector/DOMEditor.cpp
+++ b/Source/core/inspector/DOMEditor.cpp
@@ -175,9 +175,10 @@ public:
 
     virtual bool perform(ExceptionState& exceptionState) OVERRIDE
     {
-        m_hadAttribute = m_element->hasAttribute(m_name);
+        const AtomicString& value = m_element->getAttribute(m_name);
+        m_hadAttribute = !value.isNull();
         if (m_hadAttribute)
-            m_oldValue = m_element->getAttribute(m_name);
+            m_oldValue = value;
         return redo(exceptionState);
     }