Move "extensions-toolbar-menu" flag expiry to M84

Bug: None
Change-Id: I311e0030a68c23060a06ea7520a2580732fa8451
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066879
Commit-Queue: Peter Boström <pbos@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Auto-Submit: Peter Boström <pbos@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743274}

chrome/browser/flag-metadata.json

@@ -2250,7 +2250,7 @@
   {
     "name": "extensions-toolbar-menu",
     "owners": [ "//extensions/OWNERS", "pbos" ],
-    "expiry_milestone": 82
+    "expiry_milestone": 84
   },
   {
     "name": "file-handling-api",