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

Change-Id: I7d1ee16697795b502c176a16755ca3433c8fdd3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2458773
Commit-Queue: Joe DeBlasio <jdeblasio@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Auto-Submit: Joe DeBlasio <jdeblasio@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814904}

chrome/browser/flag-metadata.json

@@ -4478,7 +4478,7 @@
   {
     "name": "treat-unsafe-downloads-as-active-content",
     "owners": [ "jdeblasio", "cthomp" ],
-    "expiry_milestone": 87
+    "expiry_milestone": 89
   },
   {
     "name": "trim-on-memory-pressure",