Update HTTPHeaderMap wrappers to use AtomicString type for header values

Source/modules/websockets/WebSocketHandshake.h

Index: Source/modules/websockets/WebSocketHandshake.h
diff --git a/Source/modules/websockets/WebSocketHandshake.h b/Source/modules/websockets/WebSocketHandshake.h
index 0860421970efe01d54be038deadb8aa4c85db2dc..835aae72c933fe223d8044fd2e10c413eb490ba2 100644
--- a/Source/modules/websockets/WebSocketHandshake.h
+++ b/Source/modules/websockets/WebSocketHandshake.h
@@ -80,12 +80,12 @@ public:
     // Returns a string indicating the reason of failure if mode() == Failed.
     String failureReason() const;
 
-    String serverWebSocketProtocol() const;
-    String serverSetCookie() const;
-    String serverSetCookie2() const;
-    String serverUpgrade() const;
-    String serverConnection() const;
-    String serverWebSocketAccept() const;
+    const AtomicString& serverWebSocketProtocol() const;
+    const AtomicString& serverSetCookie() const;
+    const AtomicString& serverSetCookie2() const;
+    const AtomicString& serverUpgrade() const;
+    const AtomicString& serverConnection() const;
+    const AtomicString& serverWebSocketAccept() const;
     String acceptedExtensions() const;
 
     const WebSocketHandshakeResponse& serverHandshakeResponse() const;