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

Source/core/html/parser/XSSAuditor.cpp

Index: Source/core/html/parser/XSSAuditor.cpp
diff --git a/Source/core/html/parser/XSSAuditor.cpp b/Source/core/html/parser/XSSAuditor.cpp
index 4c12a0b89f11cb40cc1d1bfcfc379500f229f7ce..1fb6a0f303e737a637155fc8c80d05edce0dc85d 100644
--- a/Source/core/html/parser/XSSAuditor.cpp
+++ b/Source/core/html/parser/XSSAuditor.cpp
@@ -263,8 +263,8 @@ void XSSAuditor::init(Document* document, XSSAuditorDelegate* auditorDelegate)
 
     String httpBodyAsString;
     if (DocumentLoader* documentLoader = document->frame()->loader().documentLoader()) {
-        DEFINE_STATIC_LOCAL(String, XSSProtectionHeader, ("X-XSS-Protection"));
-        String headerValue = documentLoader->response().httpHeaderField(XSSProtectionHeader);
+        DEFINE_STATIC_LOCAL(const AtomicString, XSSProtectionHeader, ("X-XSS-Protection", AtomicString::ConstructFromLiteral));
+        const AtomicString& headerValue = documentLoader->response().httpHeaderField(XSSProtectionHeader);
         String errorDetails;
         unsigned errorPosition = 0;
         String reportURL;