Extend the lifetime of the app banner bypass flag.

This flag will likely be able to be removed soon as new improvements to
devtools are made. For now, keep it around.

BUG: None
Change-Id: I8cb531cbcea8260582fce5cbde4cae0ad3e22611
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1921018
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Maggie Cai <mxcai@chromium.org>
Auto-Submit: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Maggie Cai <mxcai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716080}

chrome/browser/flag-metadata.json

@@ -387,7 +387,7 @@
   {
     "name": "bypass-app-banner-engagement-checks",
     "owners": [ "dominickn" ],
-    "expiry_milestone": 78
+    "expiry_milestone": 84
   },
   {
     "name": "calculate-native-win-occlusion",