[Nickname Management] Modify fieldtrial testing config

Bug: 1082013
Change-Id: Ic6736d65ff962a400cd0cd714ca27a25200346c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401849Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Siyu An <siyua@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805842}

testing/variations/fieldtrial_testing_config.json

@@ -299,6 +299,7 @@
                 "android",
                 "android_webview",
                 "chromeos",
+                "ios",
                 "linux",
                 "mac",
                 "windows"