[Screenshot] Move experiment expiry.

Change-Id: I207a3b1bf4f6b80dbdde455d80fa5f51899c4214
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548521Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829042}

chrome/browser/flag-metadata.json

@@ -499,7 +499,7 @@
   {
     "name": "chrome-share-screenshot",
     "owners": [ "jeffreycohen", "kristipark" ],
-    "expiry_milestone": 88
+    "expiry_milestone": 90
   },
   {
     "name": "chrome-sharing-hub",