[translate] Add translate ranker model loader.

components/translate/core/browser/proto/ranker_model.proto

Index: components/translate/core/browser/proto/ranker_model.proto
diff --git a/components/translate/core/browser/proto/ranker_model.proto b/components/translate/core/browser/proto/ranker_model.proto
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+// 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, 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, then there
+  // is no need to refresh the model.
+  optional string source = 1;
+
+  // The timestamp at which this model was download..

[hamelphi, 2017/01/20 18:44:18]

downloaded, remove one '.'

[Roger McFarlane (Chromium), 2017/02/08 23:08:09]

Done.

+  optional int64 last_modified_sec = 2;
+
+  // The number of seconds after which this model should be considered expired.
+  // Defaults to 30 days, in seconds.
+  optional int64 cache_duration_sec = 3 [default = 2592000];

[hamelphi, 2017/01/20 18:44:18]

What does it mean for a model to be expired? Is the model cache deleted?
Does that imply a new download of the model after 30 days?
Does 0 mean no time limit?
Do we really want the models to expire by default?
Is this controlled by Finch?

For instance, for stable models such as CLD, we may not know in advance what the
expect time of life of the model is. And if those models are big, we may not
want to retrigger a download unless needed.

[Roger McFarlane (Chromium), 2017/02/08 23:08:09]

Updated thee documentation and dropped the default.

This is optional functionality, which for translate ranker we probably won't
use, since we plan to publish immutable models which never expire and move the
URL in Finch. 

I can pull this out with the assumption that YAGNI and if we need it eventually,
it's easy to put back in.

+}
+
+// Defines an envelope/wrapper for general models.
+message RankerModel {
+  // Metadata.
+  optional RankerModelMetadata metadata = 1;
+
+  oneof model_type { TranslateRankerModel translate = 2; }

[hamelphi, 2017/01/20 18:44:18]

This could be only 'model'. I find that model_type does not reflect the fact
that this actually is the proto of the model.

We could have a model_type, id, type or name in the metadata section though. It
could be useful to have some kind of name or key we can refer to in finch, so we
could pass a dictionary of models, and also when we want to fetch the model
internally.

[Roger McFarlane (Chromium), 2017/02/08 23:08:08]

That's not how the proto API works. You don't access the members through the
oneof name, you access them directly and switch on the oneof name.

I.e., it's the prefix of the switch case value you use to check which of the
member fields should be valid; it becomes an enum.
Added a name and label.

+}