Expose API to detect when captive portal probe was blocked on Android

net/android/network_library.cc

Index: net/android/network_library.cc
diff --git a/net/android/network_library.cc b/net/android/network_library.cc
index 939319cd0b2dcf624bdde5674551a12ea65406be..6612381853f4d7082342393cec62bc97e6058a08 100644
--- a/net/android/network_library.cc
+++ b/net/android/network_library.cc
@@ -127,6 +127,12 @@ bool GetIsRoaming() {
       base::android::GetApplicationContext());
 }
 
+bool GetIsCaptivePortal() {
+  return Java_AndroidNetworkLibrary_getIsCaptivePortal(
+      base::android::AttachCurrentThread(),
+      base::android::GetApplicationContext());
+}
+
 std::string GetWifiSSID() {
   return base::android::ConvertJavaStringToUTF8(
       Java_AndroidNetworkLibrary_getWifiSSID(