Enable inline replies for notifications by default

This was approved in the following blink-dev thread:
https://groups.google.com/a/chromium.org/d/topic/blink-dev/PNTQCliD2oA/discussion

Bug: 599859, 834580
Change-Id: I5419f4b804d864f7837487ec881fb5542d9304df
Reviewed-on: https://chromium-review.googlesource.com/1169477Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582114}

third_party/WebKit/LayoutTests/virtual/stable/http/tests/serviceworker/webexposed/global-interface-listing-service-worker-expected.txt

@@ -696,6 +696,7 @@ interface NotificationEvent : ExtendableEvent
     attribute @@toStringTag
     getter action
     getter notification
+    getter reply
     method constructor
 interface OffscreenCanvas : EventTarget
     attribute @@toStringTag

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -815,7 +815,7 @@
     },
     {
       name: "NotificationInlineReplies",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "Notifications",