Enable 'Password Leak Detection' on the waterfall for iOS.

Bug: 986298
Change-Id: I0265753725783fb569783ba6006dec01d7793851
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1893881
Auto-Submit: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712098}

testing/variations/fieldtrial_testing_config.json

@@ -4315,7 +4315,8 @@
                 "chromeos",
                 "linux",
                 "mac",
-                "windows"
+                "windows",
+                "ios"
             ],
             "experiments": [
                 {