Change net's predictor_tab_helper to use the new content Navigation API

testing/variations/fieldtrial_testing_config_linux.json

 {
     "AsyncDNS": [
         {
             "group_name": "AsyncDNSA"
         }
     ],
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
@@ skipping 78 matching lines @@
     "PageRevisitInstrumentation": [
         {
             "group_name": "Enabled"
         }
     ],
     "PasswordBranding": [
         {
             "group_name": "SmartLockBrandingSavePromptOnly"
         }
     ],
+    "PreconnectMore": [
+        {
+            "group_name": "PreconnectMoreAllFrames"

[Alexei Svitkine (slow), 2016/03/21 19:42:12]

This doesn't actually enable the features unless you specify "enable_features":

[Charlie Harrison, 2016/03/22 16:26:36]

Ah ok. Updated.

+        },
+        {
+            "group_name": "PreconnectMoreMainFrameOnly"
+        }
+    ],
     "QUIC": [
         {
             "group_name": "Enabled"
         }
     ],
     "RefreshTokenDeviceId": [
         {
             "group_name": "Enabled"
         }
     ],
@@ skipping 139 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }