Extending Assistant voice search flag expiration

Bug: 1117271
Change-Id: I0cfa8867edb70df45f2ed382aa745cc29390d160
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2538533Reviewed-by: Filip Gorski <fgorski@chromium.org>
Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828446}

chrome/browser/flag-metadata.json

@@ -3417,7 +3417,7 @@
   {
     "name": "omnibox-assistant-voice-search",
     "owners": [ "wylieb", "chrome-omnibox-team@google.com" ],
-    "expiry_milestone": 88
+    "expiry_milestone": 90
   },
   {
     "name": "omnibox-bookmark-paths",