Extend expiry of allow-sync-xhr flag.

This features removal has been pushed back to M93. This CL extends the
flag to correspond to the new removal timeline.

Bug: 1003101
Change-Id: Iebe77bc047034f4f6f6ae1c4538a2f85236c33f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613562Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Katie Dillon <kdillon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841618}

chrome/browser/flag-metadata.json

@@ -92,7 +92,7 @@
   {
     "name": "allow-sync-xhr-in-page-dismissal",
     "owners": [ "kdillon@chromium.org" ],
-    "expiry_milestone": 88
+    "expiry_milestone": 93
   },
   {
     "name": "android-default-browser-promo",