libchromeos: Move Dictionary class into its own header file

Moved chromeos::dbus_utils::Dictionary type alias into its
own header file (chromeos/dictionary.h) and promoting it to the
top level namespace (chromeos).

So now it becomes chromeos::VariantDictionary and it is now
independent from D-Bus data serialization.

BUG=None
TEST=FEATURE=test emerge-link libchromeos buffet

Change-Id: Ibad63fd4168d77b9a4b80c7800016510f87ae33d
Reviewed-on: https://chromium-review.googlesource.com/219413
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/commands/dbus_command_proxy.h b/buffet/commands/dbus_command_proxy.h
index eb37d32..553470e 100644
--- a/buffet/commands/dbus_command_proxy.h
+++ b/buffet/commands/dbus_command_proxy.h
@@ -46,7 +46,7 @@
   chromeos::dbus_utils::ExportedProperty<std::string> id_;
   chromeos::dbus_utils::ExportedProperty<std::string> status_;
   chromeos::dbus_utils::ExportedProperty<int32_t> progress_;
-  chromeos::dbus_utils::ExportedProperty<chromeos::dbus_utils::Dictionary>
+  chromeos::dbus_utils::ExportedProperty<chromeos::VariantDictionary>
       parameters_;
 
   // Handles calls to org.chromium.Buffet.Command.SetProgress(progress).