ServiceWorker: Fix conversion from Request headers to WebServiceWorkerRequest headers

public/platform/WebServiceWorkerRequest.h

Index: public/platform/WebServiceWorkerRequest.h
diff --git a/public/platform/WebServiceWorkerRequest.h b/public/platform/WebServiceWorkerRequest.h
index 89a45ae0c0859f46a40dd5f2b47d8b0dc13ccd31..ed46a205f6518bc687550c552c42bd8f49a780a1 100644
--- a/public/platform/WebServiceWorkerRequest.h
+++ b/public/platform/WebServiceWorkerRequest.h
@@ -47,6 +47,11 @@ public:
     WebString method() const;
 
     void setHeader(const WebString& key, const WebString& value);
+
+    // If the key already exists, the value is appended to the existing value
+    // with a comma delimiter between them.
+    void appendHeader(const WebString& key, const WebString& value);
+
     void visitHTTPHeaderFields(WebHTTPHeaderVisitor*) const;
 
     void setBlob(const WebString& uuid, long long size);