Fix classes that have too many virtuals for inline constructors.

device/nfc/nfc_adapter.h

Index: device/nfc/nfc_adapter.h
diff --git a/device/nfc/nfc_adapter.h b/device/nfc/nfc_adapter.h
index 455b14289b823e1d86924ec8867a2775b6c6c3a5..65d15f62838629b4845b4448ef28cf4e871082e3 100644
--- a/device/nfc/nfc_adapter.h
+++ b/device/nfc/nfc_adapter.h
@@ -85,6 +85,9 @@ class NfcAdapter : public base::RefCounted<NfcAdapter> {
     // Called when the remote NFC adapter |peer| is no longer known by the
     // adapter |adapter|. |peer| should not be cached.
     virtual void PeerLost(NfcAdapter* adapter, NfcPeer* peer) {}
+
+   protected:
+    Observer() = default;
   };
 
   // The ErrorCallback is used by methods to asynchronously report errors.