buffet: Moved LoadJsonDict() function into buffet/utils.h

Moved LoadJsonDict from CommandManager class into a separate
header file as a stand-alone function so that it can be
used by itself.

This function will come in handy when implementing device
state management.

BUG=chromium:415364
TEST=FEATURES=test emerge-link buffet

Change-Id: Ie818a811989d82e4b092e399ca136e15276ab453
Reviewed-on: https://chromium-review.googlesource.com/219134
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/device_registration_info.cc b/buffet/device_registration_info.cc
index 0368c83..2f2ddac 100644
--- a/buffet/device_registration_info.cc
+++ b/buffet/device_registration_info.cc
@@ -20,11 +20,11 @@
 #include "buffet/commands/command_manager.h"
 #include "buffet/device_registration_storage_keys.h"
 #include "buffet/storage_impls.h"
+#include "buffet/utils.h"
 
 const char buffet::kErrorDomainOAuth2[] = "oauth2";
 const char buffet::kErrorDomainGCD[] = "gcd";
 const char buffet::kErrorDomainGCDServer[] = "gcd_server";
-const char buffet::kErrorDomainBuffet[] = "buffet";
 
 namespace buffet {
 namespace storage_keys {