ResourceLoadScheduler: adopt launch candidate configuration

Field study indicates that the bg_limit_3_1 group shows the best
result. This patch adopt the same configuration for perftests to see
if there is a overlooked regressed scenario with the configuration.

Bug: 729954
Change-Id: Ied7a28851e4ebc86e65916c757dbf336a72863ff
Reviewed-on: https://chromium-review.googlesource.com/964036
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543561}

testing/variations/fieldtrial_testing_config.json

@@ -3063,10 +3063,10 @@
             ],
             "experiments": [
                 {
-                    "name": "Enabled_bg_limit_8_4",
+                    "name": "Enabled_bg_limit_3_1",
                     "params": {
-                        "bg_limit": "8",
-                        "bg_sub_limit": "4"
+                        "bg_limit": "3",
+                        "bg_sub_limit": "1"
                     },
                     "enable_features": [
                         "ResourceLoadScheduler"