predictors: Remove unused field from the resource_prefetch_predictor database.

chrome/browser/predictors/resource_prefetch_predictor.proto

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Protocol buffers used for storing in SQLite.
 // CAUTION: When any change is done here, bump kDatabaseVersion in
 //          resource_prefetch_predictor.h.
 
 syntax = "proto2";
 
@@ skipping 33 matching lines @@
 
   // Mirrors net::RequestPriority
   enum Priority {
     REQUEST_PRIORITY_IDLE = 0;
     REQUEST_PRIORITY_LOWEST = 1;
     REQUEST_PRIORITY_LOW = 2;
     REQUEST_PRIORITY_MEDIUM = 3;
     REQUEST_PRIORITY_HIGHEST = 4;
   }
 
-  optional string primary_key = 1;
-  optional string resource_url = 2;
-  optional ResourceType resource_type = 3;
-  optional uint32 number_of_hits = 4;
-  optional uint32 number_of_misses = 5;
-  optional uint32 consecutive_misses = 6;
-  optional double average_position = 7;
-  optional Priority priority = 8;
-  optional bool has_validators = 9;
-  optional bool always_revalidate = 10;
+  optional string resource_url = 1;
+  optional ResourceType resource_type = 2;
+  optional uint32 number_of_hits = 3;
+  optional uint32 number_of_misses = 4;
+  optional uint32 consecutive_misses = 5;
+  optional double average_position = 6;
+  optional Priority priority = 7;
+  optional bool has_validators = 8;
+  optional bool always_revalidate = 9;
 }