Set enable-reader-mode flag to expire in M82.

Change-Id: I47d8314c480b1a3e64a5f8ebb63c97c0af9bf516
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1903759Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Cr-Commit-Position: refs/heads/master@{#713867}

chrome/browser/flag-metadata.json

@@ -1674,7 +1674,7 @@
   {
     "name": "enable-reader-mode",
     "owners": [ "gilmanmh@google.com" ],
-    "expiry_milestone": 78
+    "expiry_milestone": 82
   },
   {
     "name": "enable-reader-mode-in-cct",