predictors: Enable SpeculativeResourcePrefetching for fieldtrial testing.

BUG=631966

Review-Url: https://codereview.chromium.org/2856893004
Cr-Commit-Position: refs/heads/master@{#469317}

testing/variations/fieldtrial_testing_config.json

@@ -2849,6 +2849,24 @@
             ]
         }
     ],
+    "SpeculativeResourcePrefetchingValidation": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Prefetching",
+                    "params": {
+                        "mode": "prefetching"
+                    },
+                    "enable_features": [
+                        "SpeculativeResourcePrefetching"
+                    ]
+                }
+            ]
+        }
+    ],
     "StrictSecureCookies": [
         {
             "platforms": [