Update PWM onboarding fieldtrial testing config with param

Bug: 983445
Change-Id: I3c5f67636397983c0765e7baf73ecb54bc45c889
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862440
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: Ioana Pandele <ioanap@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705938}

testing/variations/fieldtrial_testing_config.json

@@ -478,7 +478,10 @@
             ],
             "experiments": [
                 {
-                    "name": "Enabled",
+                    "name": "EnabledWithSafetyStory",
+                    "params": {
+                        "story": "safety"
+                    },
                     "enable_features": [
                         "PasswordManagerOnboardingAndroid"
                     ]