[Chromecast] Use base::FeatureList to control features.

chromecast/base/cast_features.h

+// Copyright 2017 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.
+
+#ifndef CHROMECAST_BASE_CAST_FEATURES_H_
+#define CHROMECAST_BASE_CAST_FEATURES_H_
+
+#include <memory>
+#include <unordered_set>
+
+#include "base/feature_list.h"
+#include "base/macros.h"
+
+namespace base {
+class DictionaryValue;
+class ListValue;
+}
+
+namespace chromecast {
+
+// Initialize the global base::FeatureListInstance, taking into account
+// overrides from DCS and the command line. |features| is read from the
+// PrefService in the browser. Thread-safe.

[halliwell, 2017/04/20 02:49:31]

I'm not sure "Thread-safe" completely characterises these functions.  It's more
just that you have to be certain this one that gets called once is finished,
then you can call GetDCSExperimentIds on any threads.

[slan, 2017/04/20 16:26:02]

You're right. Done.

+void InitializeFeatureList(const base::DictionaryValue& dcs_features,
+                           const base::ListValue& dcs_experiment_ids,
+                           const std::string& cmd_line_enable_features,

[halliwell, 2017/04/20 02:49:31]

nit, include string

[slan, 2017/04/20 16:26:02]

Done.

+                           const std::string& cmd_line_disable_features);
+
+// Given a dictionary of features, create a copy that is ready to be persisted
+// to disk. Encodes all values as strings,  which is how the FieldTrial
+// classes expect the param data.
+std::unique_ptr<base::DictionaryValue> GetOverriddenFeaturesForStorage(
+    const base::DictionaryValue& features);
+
+// Query the set of experiment ids set for this run. Intended only for metrics
+// reporting. Must be called after InitializeFeatureList. Thread-safe.
+std::unordered_set<int32_t> GetDCSExperimentIds();

[halliwell, 2017/04/20 02:49:31]

include <cstdint>

[halliwell, 2017/04/20 02:49:31]

how big is this set?  Are users expecting to modify it?  If not, should return
const ref.

[slan, 2017/04/20 16:26:02]

Done.

[slan, 2017/04/20 16:26:02]

This set is read-only. It will only ever be as large as the number of concurrent
experiments running on a single device. In practice, this number will be < 5. 

I remember reading somewhere that a copy should be returned for small container,
but I can't find the documentation to back that up, so... Done.

+
+// Reset static state to ensure clean unittests. For tests only!
+void ResetCastFeaturesForTest();
+
+}  // namespace chromecast
+
+#endif  // CHROMECAST_BASE_CAST_FEATURES_H_