Read later: update flag expiry milestone.

Bug: 1109316
Change-Id: Id8f9c23c6b1526800ae7366c68cc4c8b4c02a23b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577484
Auto-Submit: Caroline Rising <corising@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834346}

chrome/browser/flag-metadata.json

@@ -4043,7 +4043,7 @@
   {
     "name": "read-later",
     "owners": [ "chrome-desktop-ui-sea@google.com", "corising" ],
-    "expiry_milestone": 90
+    "expiry_milestone": 94
   },
   {
     "name": "reader-mode-heuristics",