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

Source/core/loader/FrameLoader.cpp

Index: Source/core/loader/FrameLoader.cpp
diff --git a/Source/core/loader/FrameLoader.cpp b/Source/core/loader/FrameLoader.cpp
index 9778420dad8c95a249dcd5cb643be3f84cd62655..81e34edcd07ec2bdb54189edc7df30b75b6e655b 100644
--- a/Source/core/loader/FrameLoader.cpp
+++ b/Source/core/loader/FrameLoader.cpp
@@ -355,7 +355,7 @@ void FrameLoader::didBeginDocument(bool dispatch)
     }
 
     if (m_documentLoader) {
-        String dnsPrefetchControl = m_documentLoader->response().httpHeaderField("X-DNS-Prefetch-Control");
+        const AtomicString& dnsPrefetchControl = m_documentLoader->response().httpHeaderField("X-DNS-Prefetch-Control");
         if (!dnsPrefetchControl.isEmpty())
             m_frame->document()->parseDNSPrefetchControlHeader(dnsPrefetchControl);