Add entry for socket pool splitting to fieldtrial_testing_config.json

Also fix the LoadingPredictor tests that failed when it was enabled.

Bug: 963480
Change-Id: I4c8d87d0d97bfd4e8e4e639921da4cd4f908ca5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1832248
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702503}

testing/variations/fieldtrial_testing_config.json

@@ -4196,6 +4196,26 @@
             ]
         }
     ],
+    "PartitionConnectionsByNetworkIsolationKey": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "PartitionConnectionsAndSSLSessions",
+                    "enable_features": [
+                        "PartitionConnectionsByNetworkIsolationKey",
+                        "PartitionSSLSessionsByNetworkIsolationKey"
+                    ]
+                }
+            ]
+        }
+    ],
     "PassiveDocumentEventListeners": [
         {
             "platforms": [