Teach the GCM Driver how to decrypt incoming messages.

components/gcm_driver/common/gcm_messages.h

Index: components/gcm_driver/common/gcm_messages.h
diff --git a/components/gcm_driver/common/gcm_messages.h b/components/gcm_driver/common/gcm_messages.h
index 8d8c5b957c0664f803cb02929e49eccce5d5a1a7..a6a071cc8c1ef37d4b29a54df131a0e347d23a47 100644
--- a/components/gcm_driver/common/gcm_messages.h
+++ b/components/gcm_driver/common/gcm_messages.h
@@ -38,6 +38,10 @@ struct GCM_DRIVER_EXPORT IncomingMessage {
   std::string collapse_key;
   std::string sender_id;
   std::string raw_data;
+
+  // Whether the contents of the message have been decrypted, and are
+  // available in |raw_data|.
+  bool decrypted;
 };
 
 }  // namespace gcm