Have HashMap<KeyType, AtomicString>::get() return a const reference

Source/core/html/HTMLElement.cpp

Index: Source/core/html/HTMLElement.cpp
diff --git a/Source/core/html/HTMLElement.cpp b/Source/core/html/HTMLElement.cpp
index 180a074eab2a9f5771fe0b475e8aba8b9b8728a1..1bc9d2563d5711caaf0988eb2f3477c15feedf01 100644
--- a/Source/core/html/HTMLElement.cpp
+++ b/Source/core/html/HTMLElement.cpp
@@ -205,10 +205,10 @@ void HTMLElement::collectStyleForPresentationAttribute(const QualifiedName& name
         Element::collectStyleForPresentationAttribute(name, value, style);
 }
 
-AtomicString HTMLElement::eventNameForAttributeName(const QualifiedName& attrName) const
+const AtomicString& HTMLElement::eventNameForAttributeName(const QualifiedName& attrName) const
 {
     if (!attrName.namespaceURI().isNull())
-        return AtomicString();
+        return nullAtom;
 
     typedef HashMap<AtomicString, AtomicString> StringToStringMap;
     DEFINE_STATIC_LOCAL(StringToStringMap, attributeNameToEventNameMap, ());
@@ -320,7 +320,7 @@ void HTMLElement::parseAttribute(const QualifiedName& name, const AtomicString&
             setTabIndexExplicitly(max(static_cast<int>(std::numeric_limits<short>::min()), min(tabindex, static_cast<int>(std::numeric_limits<short>::max()))));
         }
     } else {
-        AtomicString eventName = eventNameForAttributeName(name);
+        const AtomicString& eventName = eventNameForAttributeName(name);
         if (!eventName.isNull())
             setAttributeEventListener(eventName, createAttributeEventListener(this, name, value));
     }