[translate] Add translate ranker model loader.

components/translate/core/browser/proto/translate_ranker_model.proto

 // Copyright (c) 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.
 //
 // Experimental Translation Assist Model to allow/suppress translation prompts.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package chrome_intelligence;
 
-// Defines a Chrome Ranker Translate model.
+// Defines a Chrome Ranker Translate Model.
 // Next tag: 12
 message TranslateRankerModel {
   // A number that identifies the version of this model.
   optional uint32 version = 1;
 
   // Defines the weights and bias of a Logistic Regression Model.
   message LogisticRegressionModel {
     optional float bias = 1;
 
     optional float accept_ratio_weight = 2;
     optional float decline_ratio_weight = 3;
     optional float ignore_ratio_weight = 4;
 
     optional float accept_count_weight = 9;
     optional float decline_count_weight = 10;
     optional float ignore_count_weight = 11;
 
     // One-hot features are encoded in the form of a map. These maps
     // each contain an element 'UNKNOWN' to use in case the key is not
     // found in the map.
     map<string, float> source_language_weight = 5;
     map<string, float> dest_language_weight = 6;
     map<string, float> country_weight = 7;
     map<string, float> locale_weight = 8;
   }
-  optional LogisticRegressionModel logistic_regression_model = 2;
+
+  oneof model_revision {
+    LogisticRegressionModel logistic_regression_model = 2;
+  }
 }