Enable NetProxyPreconnects experiment by default

testing/variations/fieldtrial_testing_config.json

 {
     "AlternateComponentUrls": [
         {
             "platforms": [
                 "android",
                 "chromeos",
                 "ios",
                 "linux",
                 "mac",
                 "win"
@@ skipping 1570 matching lines @@
             "experiments": [
                 {
                     "name": "Enabled",
                     "enable_features": [
                         "PrioritySupportedRequestsDelayable"
                     ]
                 }
             ]
         }
     ],
-    "NetProxyPreconnects": [
-        {
-            "platforms": [
-                "android",
-                "chromeos",
-                "ios",
-                "linux",
-                "mac",
-                "win"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "params": {
-                        "restrict_to_one_preconnect_for_proxies": "true"
-                    }
-                }
-            ]
-        }
-    ],
     "NetworkQualityEstimator": [
         {
             "platforms": [
                 "android",
                 "win"
             ],
             "experiments": [
                 {
                     "name": "Enabled",
                     "params": {
@@ skipping 1728 matching lines @@
                 {
                     "name": "Enabled",
                     "enable_features": [
                         "use-new-media-cache"
                     ]
                 }
             ]
         }
     ]
 }