Remove RuntimeEnabled from NotificationEventInit

This is needed to complete the shipping of action buttons.
It was accidentally not included in https://crrev.com/358304
(or https://codereview.chromium.org/1487763003).

See also https://crbug.com/546006.

BUG=513671

Review URL: https://codereview.chromium.org/1489773003

Cr-Commit-Position: refs/heads/master@{#362423}

third_party/WebKit/Source/modules/notifications/NotificationEventInit.idl

@@ -6,5 +6,5 @@
 
 dictionary NotificationEventInit : ExtendableEventInit {
     Notification notification;
-    [RuntimeEnabled=NotificationExperimental] DOMString action = "";
+    DOMString action = "";
 };