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

Source/core/fetch/CrossOriginAccessControl.cpp

Index: Source/core/fetch/CrossOriginAccessControl.cpp
diff --git a/Source/core/fetch/CrossOriginAccessControl.cpp b/Source/core/fetch/CrossOriginAccessControl.cpp
index b9433ac057ac9715de2dbba777664ea0ecd75d38..90c032352de04f55b34a5a3705dbaa815f5275d4 100644
--- a/Source/core/fetch/CrossOriginAccessControl.cpp
+++ b/Source/core/fetch/CrossOriginAccessControl.cpp
@@ -141,7 +141,7 @@ bool passesAccessControlCheck(const ResourceResponse& response, StoredCredential
 
     // A wildcard Access-Control-Allow-Origin can not be used if credentials are to be sent,
     // even with Access-Control-Allow-Credentials set to true.
-    const String& accessControlOriginString = response.httpHeaderField(accessControlAllowOrigin);
+    const AtomicString& accessControlOriginString = response.httpHeaderField(accessControlAllowOrigin);
     if (accessControlOriginString == "*" && includeCredentials == DoNotAllowStoredCredentials)
         return true;
 
@@ -162,7 +162,7 @@ bool passesAccessControlCheck(const ResourceResponse& response, StoredCredential
     }
 
     if (includeCredentials == AllowStoredCredentials) {
-        const String& accessControlCredentialsString = response.httpHeaderField(accessControlAllowCredentials);
+        const AtomicString& accessControlCredentialsString = response.httpHeaderField(accessControlAllowCredentials);
         if (accessControlCredentialsString != "true") {
             errorDescription = "Credentials flag is 'true', but the 'Access-Control-Allow-Credentials' header is '" + accessControlCredentialsString + "'. It must be 'true' to allow credentials.";
             return false;