Use Android callback API to obtain cellular signal strength

net/nqe/network_quality_observation.h

 // Copyright 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.
 
 #ifndef NET_NQE_NETWORK_QUALITY_OBSERVATION_H_
 #define NET_NQE_NETWORK_QUALITY_OBSERVATION_H_
 
+#include <stdint.h>
+
 #include <vector>
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
+#include "base/optional.h"
 #include "base/time/time.h"
 #include "net/base/net_export.h"
 #include "net/nqe/network_quality_observation_source.h"
 
 namespace net {
 
 namespace nqe {
 
 namespace internal {
 
 // Records observations of network quality metrics (such as round trip time
 // or throughput), along with the time the observation was made. Observations
 // can be made at several places in the network stack, thus the observation
 // source is provided as well. ValueType must be numerical so that statistics
 // such as median, average can be computed.
 template <typename ValueType>
 struct NET_EXPORT_PRIVATE Observation {
   Observation(const ValueType& value,
               base::TimeTicks timestamp,
-              int32_t signal_strength_dbm,
+              const base::Optional<int32_t>& signal_strength,
               NetworkQualityObservationSource source)
       : value(value),
         timestamp(timestamp),
-        signal_strength_dbm(signal_strength_dbm),
+        signal_strength(signal_strength),
         source(source) {
     DCHECK(!timestamp.is_null());
   }
   ~Observation() {}
 
   // Value of the observation.
   const ValueType value;
 
   // Time when the observation was taken.
   const base::TimeTicks timestamp;
 
-  // Signal strength (in dBm) when the observation was taken. Set to INT32_MIN
-  // if the signal strength is unavailable.
-  const int32_t signal_strength_dbm;
+  // Signal strength when the observation was taken.
+  const base::Optional<int32_t> signal_strength;
 
   // The source of the observation.
   const NetworkQualityObservationSource source;
 };
 
 }  // namespace internal
 
 }  // namespace nqe
 
 }  // namespace net
 
 #endif  // NET_NQE_NETWORK_QUALITY_OBSERVATION_H_