buffet: Organize storage keys by relative lifetime

This has no functional effect but makes things a little more readable.

BUG=None
TEST=Unittests

Change-Id: Iaeb1536e3aeb622a899c91e58b0fd9b3528ce0a2
Reviewed-on: https://chromium-review.googlesource.com/247502
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>

buffet/device_registration_info.cc

diff --git a/buffet/device_registration_info.cc b/buffet/device_registration_info.cc
index fb59f3b..07b902b 100644
--- a/buffet/device_registration_info.cc
+++ b/buffet/device_registration_info.cc
@@ -35,16 +35,17 @@
 namespace buffet {
 namespace storage_keys {
 
-// Persistent keys
+// Statically configured fields
+const char kApiKey[]        = "api_key";
 const char kClientId[]      = "client_id";
 const char kClientSecret[]  = "client_secret";
-const char kApiKey[]        = "api_key";
-const char kRefreshToken[]  = "refresh_token";
-const char kDeviceId[]      = "device_id";
 const char kOAuthURL[]      = "oauth_url";
 const char kServiceURL[]    = "service_url";
+// Credentials related to a particular registration.
+const char kDeviceId[]      = "device_id";
+const char kRefreshToken[]  = "refresh_token";
 const char kRobotAccount[]  = "robot_account";
-// Transient keys
+// Fields in our device metadata.
 const char kDeviceKind[]    = "device_kind";
 const char kName[]          = "name";
 const char kDisplayName[]   = "display_name";