Use Android callback API to obtain cellular signal strength

net/nqe/network_quality_estimator.h

 // Copyright 2015 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 NET_NQE_NETWORK_QUALITY_ESTIMATOR_H_
 #define NET_NQE_NETWORK_QUALITY_ESTIMATOR_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 591 matching lines @@
 
   // Current estimate of the network quality.
   nqe::internal::NetworkQuality network_quality_;
 
   // Current effective connection type. It is updated on connection change
   // events. It is also updated every time there is network traffic (provided
   // the last computation was more than
   // |effective_connection_type_recomputation_interval_| ago).
   EffectiveConnectionType effective_connection_type_;
 
-  // Last known value of the wireless signal strength. Set to INT32_MIN if
-  // unavailable. |signal_strength_dbm_| is reset to INT32_MIN on connection
-  // change events.
-  int32_t signal_strength_dbm_;
+  // Last known value of the wireless signal strength level. If the signal
+  // strength level is available, the value is set to between 0 and 4, both
+  // inclusive. If the value is unavailable, |signal_strength_| has null value.
+  base::Optional<int32_t> signal_strength_;
 
-  // Minimum and maximum signal strength (in dBm) observed since last connection
+  // Minimum and maximum signal strength level observed since last connection
   // change. Updated on connection change and main frame requests.
-  int32_t min_signal_strength_since_connection_change_;
-  int32_t max_signal_strength_since_connection_change_;
+  base::Optional<int32_t> min_signal_strength_since_connection_change_;
+  base::Optional<int32_t> max_signal_strength_since_connection_change_;
 
   // Stores the qualities of different networks.
   std::unique_ptr<nqe::internal::NetworkQualityStore> network_quality_store_;
 
   base::ThreadChecker thread_checker_;
 
   // Manages the writing of events to the net log.
   nqe::internal::EventCreator event_creator_;
 
   // Vector that contains observation sources that should not be used when
   // computing the estimate at HTTP layer.
   const std::vector<NetworkQualityObservationSource>
       disallowed_observation_sources_for_http_;
 
   // Vector that contains observation sources that should not be used when
   // computing the estimate at transport layer.
   const std::vector<NetworkQualityObservationSource>
       disallowed_observation_sources_for_transport_;
 
   NetLogWithSource net_log_;
 
   base::WeakPtrFactory<NetworkQualityEstimator> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(NetworkQualityEstimator);
 };
 
 }  // namespace net
 
 #endif  // NET_NQE_NETWORK_QUALITY_ESTIMATOR_H_