Add Precache to fieldtrial testing config.

testing/variations/fieldtrial_testing_config_android.json

 {
     "EnhancedBookmarks": [
         {
             "group_name": "EnableEnhancedBookmarks",
             "params": {
                 "id": "Fake_id_to_trigger_experiment_code"
             }
         },
         {
             "group_name": "EnableEnhancedBookmarksGrid",
@@ skipping 221 matching lines @@
             "group_name": "Enabled"
         }
     ],
     "VarationsServiceControl": [
         {
             "group_name": "Interval_30min",
             "params": {
                 "fetch_period_min": "30"
             }
         }
+    ],
+    "Precache": [
+        {
+            "group_name": "Enabled"
+        }
     ]
 }