Use Android callback API to obtain cellular signal strength

net/nqe/network_quality_observation.h

Index: net/nqe/network_quality_observation.h
diff --git a/net/nqe/network_quality_observation.h b/net/nqe/network_quality_observation.h
index 91d163303ba4802c00c1b66cbda4a422aee3d383..f6b1cb88e06ccd15bab2eea35aff221c30d37991 100644
--- a/net/nqe/network_quality_observation.h
+++ b/net/nqe/network_quality_observation.h
@@ -5,10 +5,13 @@
 #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"
@@ -28,11 +31,11 @@ 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());
   }
@@ -44,9 +47,8 @@ struct NET_EXPORT_PRIVATE Observation {
   // 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;