Extend expiration of new-shortcut-mapping to 90

Bug: 1060690
Change-Id: Ic13dd8260154b6982b2913a9cb3551699c23da26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419925Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809646}

chrome/browser/flag-metadata.json

@@ -3153,7 +3153,7 @@
   {
     "name": "new-shortcut-mapping",
     "owners": [ "oshima", "afakhry" ],
-    "expiry_milestone": 86
+    "expiry_milestone": 90
   },
   {
     "name": "new-tabstrip-animation",