Use UUID for ServiceWorker Client identifier (1/3, blink)

Source/modules/serviceworkers/ServiceWorkerClient.h

Index: Source/modules/serviceworkers/ServiceWorkerClient.h
diff --git a/Source/modules/serviceworkers/ServiceWorkerClient.h b/Source/modules/serviceworkers/ServiceWorkerClient.h
index 0b493032b9fc00b57c9886dcb23c0f0821abbb88..0c97d16516887b6c7a3a66d406aed8a5e66a4afb 100644
--- a/Source/modules/serviceworkers/ServiceWorkerClient.h
+++ b/Source/modules/serviceworkers/ServiceWorkerClient.h
@@ -34,9 +34,11 @@ protected:
     explicit ServiceWorkerClient(const WebServiceWorkerClientInfo&);
 
     unsigned id() const { return m_id; }
+    String uuid() const { return m_uuid; }
 
 private:
-    unsigned m_id;
+    unsigned m_id; // FIXME: Deprecate this when we switch to UUID.
+    String m_uuid;
     String m_url;
 };