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

Source/core/xml/XMLHttpRequest.h

Index: Source/core/xml/XMLHttpRequest.h
diff --git a/Source/core/xml/XMLHttpRequest.h b/Source/core/xml/XMLHttpRequest.h
index dfb585e3504e66a475c303ff0bcd54d095651ba2..5c02766b3a3d89043e78b82a794e40aa53640a54 100644
--- a/Source/core/xml/XMLHttpRequest.h
+++ b/Source/core/xml/XMLHttpRequest.h
@@ -113,7 +113,7 @@ public:
     void setRequestHeader(const AtomicString& name, const AtomicString& value, ExceptionState&);
     void overrideMimeType(const AtomicString& override);
     String getAllResponseHeaders(ExceptionState&) const;
-    AtomicString getResponseHeader(const AtomicString& name, ExceptionState&) const;
+    const AtomicString& getResponseHeader(const AtomicString& name, ExceptionState&) const;
     ScriptString responseText(ExceptionState&);
     ScriptString responseJSONSource();
     Document* responseXML(ExceptionState&);
@@ -164,7 +164,7 @@ private:
     bool initSend(ExceptionState&);
     void sendBytesData(const void*, size_t, ExceptionState&);
 
-    AtomicString getRequestHeader(const AtomicString& name) const;
+    const AtomicString& getRequestHeader(const AtomicString& name) const;
     void setRequestHeaderInternal(const AtomicString& name, const AtomicString& value);
 
     // Changes m_state and dispatches a readyStateChange event if new m_state