Loading Predictor: Add field trial config for prefetching.

To prepare for testing on the Beta channel.

Bug: 1092329
Change-Id: Ibc0fe7ff8548f1c20d52a88b116b86246ac104ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2452089Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814478}

testing/variations/fieldtrial_testing_config.json

@@ -4004,15 +4004,16 @@
             ]
         }
     ],
-    "LoadingPredictorUseOptimizationGuide": [
+    "LoadingPredictorUseOptimizationGuideM86": [
         {
             "platforms": [
                 "android"
             ],
             "experiments": [
                 {
-                    "name": "Enabled_AllOn",
+                    "name": "Enabled_PrefetchAlways",
                     "enable_features": [
+                        "LoadingPredictorPrefetch",
                         "LoadingPredictorUseLocalPredictions",
                         "LoadingPredictorUseOptimizationGuide"
                     ]