Use StringBuilder::appendLiteral() / StringBuilder::append(char) when possible

Source/modules/websockets/DOMWebSocket.cpp

Index: Source/modules/websockets/DOMWebSocket.cpp
diff --git a/Source/modules/websockets/DOMWebSocket.cpp b/Source/modules/websockets/DOMWebSocket.cpp
index 6f42581ee6a1a363af76012fe2a2ab068c855348..43d4d31973ee2af896a26305b4509d8907175069 100644
--- a/Source/modules/websockets/DOMWebSocket.cpp
+++ b/Source/modules/websockets/DOMWebSocket.cpp
@@ -188,7 +188,7 @@ static String encodeSubprotocolString(const String& protocol)
         if (protocol[i] < 0x20 || protocol[i] > 0x7E)
             builder.append(String::format("\\u%04X", protocol[i]));
         else if (protocol[i] == 0x5c)
-            builder.append("\\\\");
+            builder.appendLiteral("\\\\");
         else
             builder.append(protocol[i]);
     }