Add param for the model fetch delay to field trials

Change-Id: I80ea401676b6cf3ca2fbb579eb663ad676f65b3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2019478
Auto-Submit: Michael Crouse <mcrouse@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735686}

testing/variations/fieldtrial_testing_config.json

@@ -4298,6 +4298,7 @@
                 {
                     "name": "Enabled",
                     "params": {
+                        "fetch_random_max_delay_secs": "60",
                         "max_effective_connection_type_for_navigation_hints_fetch": "4G"
                     },
                     "enable_features": [