[MIX-DL] Bump flag expiry until after rollout complete.

Change-Id: I6fb443e529da1a09eb607972e107d2f6e260fb35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2091590
Commit-Queue: Joe DeBlasio <jdeblasio@chromium.org>
Commit-Queue: Christopher Thompson <cthomp@chromium.org>
Auto-Submit: Joe DeBlasio <jdeblasio@chromium.org>
Reviewed-by: Christopher Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747787}

chrome/browser/flag-metadata.json

@@ -3553,7 +3553,7 @@
   {
     "name": "treat-unsafe-downloads-as-active-content",
     "owners": [ "jdeblasio", "cthomp" ],
-    "expiry_milestone": 82
+    "expiry_milestone": 87
   },
   {
     "name": "trim-on-all-frames-frozen",