UMA : Fixed a missing param in fieldtrial testing config

Bug: 857161
Change-Id: I2e9b78fad243612aefddfbd4b6dcce1bb4f1736e
Reviewed-on: https://chromium-review.googlesource.com/1117370Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570904}

testing/variations/fieldtrial_testing_config.json

@@ -211,6 +211,7 @@
                     "params": {
                         "availability": "any",
                         "event_trigger": "name:download_infobar_download_continuing_iph_trigger;comparator:==0;window:90;storage:360",
+                        "event_used": "name:download_home_opened;comparator:any;window:90;storage:360",
                         "session_rate": "<1"
                     },
                     "enable_features": [
@@ -231,6 +232,7 @@
                     "params": {
                         "availability": "any",
                         "event_trigger": "name:download_infobar_downloads_are_faster_iph_trigger;comparator:==0;window:90;storage:360",
+                        "event_used": "name:download_completed;comparator:any;window:90;storage:360",
                         "session_rate": "<1"
                     },
                     "enable_features": [