Merge the value of the push subscription id into the endpoint.

Source/modules/push_messaging/PushSubscription.idl

Index: Source/modules/push_messaging/PushSubscription.idl
diff --git a/Source/modules/push_messaging/PushSubscription.idl b/Source/modules/push_messaging/PushSubscription.idl
index 6b789b5bc8cad6e7b8ea0f8c58f1995646cb8f3c..0800cd206c7b1f7219806f3f2bd6ec2bdbeb9317 100644
--- a/Source/modules/push_messaging/PushSubscription.idl
+++ b/Source/modules/push_messaging/PushSubscription.idl
@@ -10,7 +10,7 @@
     RuntimeEnabled=PushMessaging,
 ] interface PushSubscription {
     readonly attribute USVString endpoint;
-    readonly attribute DOMString subscriptionId;
+    [DeprecateAs=PushSubscriptionId] readonly attribute DOMString subscriptionId;
     [CallWith=ScriptState] Promise<boolean> unsubscribe();
 
     serializer = { attribute };