Modified fieldtrial config for QuietNotificationPrompts

Added "crowd_deny_hold_back_chance" parameter, for the upcoming
experiment stage http://cl/297812954.

Bug: 103202
Change-Id: I26fb790cb1a4a61ec555b4a20049a7bf57f9ff00
TBR: mpearson@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2089731
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747628}

testing/variations/fieldtrial_testing_config.json

@@ -4223,6 +4223,7 @@
                     "name": "EnableWithAdaptiveActivation",
                     "params": {
                         "availability": "any",
+                        "crowd_deny_hold_back_chance": "0.3",
                         "enable_adaptive_activation": "true",
                         "enable_crowd_deny_triggering": "true",
                         "event_trigger": "name:tabgroups_dummy;comparator:any;window:1;storage:1",