Extend new-usb-backend flag expiration to M-90

This flag is shipping on Windows enabled-by-default in M-87 so I would like to keep it available in M-88 for debugging purposes. On macOS this flag is still disabled-by-default so I am extending the expiration through M-90 to give us time to complete the work necessary to enable it everywhere.

Bug: 637404, 1096743
Change-Id: I50f82a79e65ea5b14c1549e0969d726d76990d48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2506349
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Matt Reynolds <mattreynolds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821904}

chrome/browser/flag-metadata.json

@@ -3282,7 +3282,7 @@
   {
     "name": "new-usb-backend",
     "owners": [ "reillyg@chromium.org" ],
-    "expiry_milestone": 87
+    "expiry_milestone": 90
   },
   {
     "name": "notification-scheduler-debug-options",