Use Android callback API to obtain cellular signal strength

net/android/cellular_signal_strength.h

Index: net/android/cellular_signal_strength.h
diff --git a/net/android/cellular_signal_strength.h b/net/android/cellular_signal_strength.h
index 4165a73da782acd0e38ba19bbd5dec5daba87f5d..24d8d32a24a240f1f0addd3eddc1000d76960df1 100644
--- a/net/android/cellular_signal_strength.h
+++ b/net/android/cellular_signal_strength.h
@@ -8,29 +8,38 @@
 #include <jni.h>
 #include <stdint.h>
 
-#include "base/compiler_specific.h"
+#include "base/android/scoped_java_ref.h"
+#include "base/macros.h"
+#include "base/optional.h"
+#include "base/threading/thread_checker.h"
 #include "net/base/net_export.h"
 
 namespace net {
 
 namespace android {
 
-namespace cellular_signal_strength {
+/*
+ * CellularSignalStrength interacts with the corresponding Java class to provide
+ * signal strength values on cellular networks. The class is not thread-safe.
+ */
+class NET_EXPORT_PRIVATE CellularSignalStrength {
+ public:
+  CellularSignalStrength();
 
-// Returns true if the signal strength (in dbM) of the currently registered
-// cellular connection is available, and sets |*signal_strength_dbm| to that
-// value.
-NET_EXPORT bool GetSignalStrengthDbm(int32_t* signal_strength_dbm)
-    WARN_UNUSED_RESULT;
+  ~CellularSignalStrength();
 
-// Returns true if the signal strength level (between 0 and 4, both inclusive)
-// of the currently registered cellular connection is available, and sets
-// |*signal_strength_level| to that value with lower value indicating lower
-// signal strength.
-NET_EXPORT bool GetSignalStrengthLevel(int32_t* signal_strength_level)
-    WARN_UNUSED_RESULT;
+  // Returns the signal strength level (between 0 and 4, both inclusive) of the
+  // currently registered cellular connection. If the value is unavailable, an
+  // empty value is returned.
+  base::Optional<int32_t> GetSignalStrengthLevel() const;
 
-}  // namespace cellular_signal_strength
+ private:
+  base::android::ScopedJavaGlobalRef<jobject> j_cellular_signal_strength_;
+
+  THREAD_CHECKER(thread_checker_);
+
+  DISALLOW_COPY_AND_ASSIGN(CellularSignalStrength);
+};
 
 }  // namespace android