buffet: Fix return type of DeviceRegistrationInfo::GetDeviceInfo

Result can't be any Value. According doc string it's a JSON object
which is DictionaryValue.

BUG=none
TEST=compile

Change-Id: I0988d4558629974ad1fc980a5e096319620b585c
Reviewed-on: https://chromium-review.googlesource.com/259660
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>

buffet/device_registration_info.h

diff --git a/buffet/device_registration_info.h b/buffet/device_registration_info.h
index 7d64d31..e8f02eb 100644
--- a/buffet/device_registration_info.h
+++ b/buffet/device_registration_info.h
@@ -24,7 +24,7 @@
 #include "buffet/xmpp/xmpp_client.h"
 
 namespace base {
-class Value;
+class DictionaryValue;
 }  // namespace base
 
 namespace chromeos {
@@ -112,7 +112,8 @@
 
   // Gets the full device description JSON object, or nullptr if
   // the device is not registered or communication failure.
-  std::unique_ptr<base::Value> GetDeviceInfo(chromeos::ErrorPtr* error);
+  std::unique_ptr<base::DictionaryValue> GetDeviceInfo(
+      chromeos::ErrorPtr* error);
 
   // Registers the device.
   //