Introduce WebServiceWorkerNetworkProvider::GetProviderID() and stop static casting

third_party/WebKit/public/platform/modules/serviceworker/WebServiceWorkerNetworkProvider.h

Index: third_party/WebKit/public/platform/modules/serviceworker/WebServiceWorkerNetworkProvider.h
diff --git a/third_party/WebKit/public/platform/modules/serviceworker/WebServiceWorkerNetworkProvider.h b/third_party/WebKit/public/platform/modules/serviceworker/WebServiceWorkerNetworkProvider.h
index 93500aa7414c48c099b73a27c89b4170181120a0..3092806b7b379d70d7417b9621f2bc3f6ce0ab78 100644
--- a/third_party/WebKit/public/platform/modules/serviceworker/WebServiceWorkerNetworkProvider.h
+++ b/third_party/WebKit/public/platform/modules/serviceworker/WebServiceWorkerNetworkProvider.h
@@ -55,6 +55,9 @@ class WebServiceWorkerNetworkProvider {
   // ServiceWorker.
   virtual bool IsControlledByServiceWorker() { return false; }
 
+  // Returns an identifier of this provider.
+  virtual int GetProviderID() const { return -1; }
+
   // Returns an identifier of the service worker controlling the document
   // associated with the WebDataSource.
   virtual int64_t ServiceWorkerID() { return -1; }