Keep the reduced-referrer-granularity about:flag around

This CL bumps the expiry milestone for the
reduced-referrer-granularity about flag to M85, so that
it can be around during an M82 rollout of the new default
referrer policy and two milestones' worth of subsequent
enterprise policy lifetime.

Change-Id: If8fe6fed2691c1dafea45bb891f15abd2b702eed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2043130
Auto-Submit: David Van Cleve <davidvc@chromium.org>
Reviewed-by: Dan McArdle <dmcardle@chromium.org>
Commit-Queue: Dan McArdle <dmcardle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743184}

chrome/browser/flag-metadata.json

@@ -3180,7 +3180,7 @@
   {
     "name": "reduced-referrer-granularity",
     "owners": [ "davidvc", "mkwst", "jochen" ],
-    "expiry_milestone": 81
+    "expiry_milestone": 83
   },
   {
     "name": "release-notes",