Extend expiry date for app grid ghosting.

Change-Id: I10df8c9fe2049e1f8ae50887f004563d1dc18bfb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636614Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664785}

chrome/browser/flag-metadata.json

@@ -721,7 +721,7 @@
   {
     "name": "enable-app-grid-ghost",
     "owners": [ "mmourgos" ],
-    "expiry_milestone": 76
+    "expiry_milestone": 78
   },
   {
     "name": "enable-app-list-search-autocomplete",