Remove support for PushSubscription.subscriptionId.

Source/modules/push_messaging/PushSubscription.h

Index: Source/modules/push_messaging/PushSubscription.h
diff --git a/Source/modules/push_messaging/PushSubscription.h b/Source/modules/push_messaging/PushSubscription.h
index 8dbfd21350ba28cec11695c90fa50a232bbb49f7..8ed44e4b17e87bda0351172a6f4536a3f49b0602 100644
--- a/Source/modules/push_messaging/PushSubscription.h
+++ b/Source/modules/push_messaging/PushSubscription.h
@@ -27,7 +27,6 @@ public:
     virtual ~PushSubscription();
 
     String endpoint() const;
-    const String& subscriptionId() const { return m_subscriptionId; }
     ScriptPromise unsubscribe(ScriptState*);
 
     ScriptValue toJSONForBinding(ScriptState*);
@@ -35,10 +34,9 @@ public:
     DECLARE_TRACE();
 
 private:
-    PushSubscription(const String& endpoint, const String& subscriptionId, ServiceWorkerRegistration*);
+    PushSubscription(const String& endpoint, ServiceWorkerRegistration*);
 
     String m_endpoint;
-    String m_subscriptionId;
     Member<ServiceWorkerRegistration> m_serviceWorkerRegistration;
 };