[translate] Add translate ranker model loader.

components/translate/core/browser/proto/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;
+
+import "translate_ranker_model.proto";
+
+package chrome_intelligence;
+
+// Metadata for a ranker model instance. This data describes how the ranker
+// model should be interpreted/used.
+message RankerModelMetadata {
+  // An identifier denoting the type or purpose of this model.
+  optional string name = 1;
+
+  // An identifier denoting the specific instance of this model. For example:
+  // "Experiment B"
+  optional string label = 2;
+
+  // An identifier, typically a URL, denoting the source from which this model
+  // was obtained. The model referenced with a given source is presumed to be
+  // immutable; this can be used as a cache control mechanism. If the currently
+  // configured model source matches the source of a cached model, and the
+  // cached model has not expired then there is no need to refresh the model.
+  optional string source = 3;
+
+  // The timestamp at which this model was downloaded. This will be set by the
+  // model loader before it caches the model to disk.
+  optional int64 last_modified_sec = 4;
+
+  // The (optional) number of seconds after which this model should be
+  // considered expired. If the value is zero or not set, then the cached
+  // instance of the model never expires. A new download can be triggered by
+  // changing the configured source URL for the model loader.
+  optional int64 cache_duration_sec = 5;
+}
+
+// Defines an envelope/wrapper for general models.
+message RankerModelProto {
+  // Metadata.
+  optional RankerModelMetadata metadata = 1;
+
+  oneof model { TranslateRankerModel translate = 2; }
+}